Feral cat I watch/feed was prego, but no signs anymore of kittens

I have a young, very petite feral kitty I call Tina (very young) that I feed along with my other transients, who, if they decide to become regulars, I try to TNR.  Most of the males that lurk around my yard and shed have been neutered over the past few years, and I have only had one female I've had spayed and released again, and I think she got run over on the road not long after.

Tina started off as what I refer to as one of my transients--I would maybe see her stop by and eat at my place every other day or so. 

For some reason, she became a regular about December, when she started sleeping in a box on my porch and actually let me pet her, and pick her up.  I was waiting to see if she hung around, and if so, I was going to take her to be spayed.  That was when I noticed she was getting round.  She is a tiny built kitty, very thin and long-legged, so she started showing really quickly. I suspect she is the sister of a little boy, Rufus, who I found a home for back in Sept 2015.  She is probably a year old if she is his sister, I'm guessing this because they are similarly built and colored, and both of them have a small kink at the end of their tail. 

Anyway, she disappeared about 2 weeks ago and didn't come by for a few days, after being a twice-a-day regular for handouts.  She finally showed up skinny and starving, so I knew she had her babies.  I watched her after she ate make her way across to the neighbor's shed, so I figured that's where she was holed up with them.  The next couple of days she ate like a horse (I fed her lots and lots because I thought she was nursing) and would go back over there.

Now she is back at my house and spending a lot of time on the porch.  I mean she lays out there for hours.  I picked her up yesterday evening, and was shocked to see she had no milk, no fur missing from her belly, it's like she has no kittens nursing on her at all.  What could have happened?  I have heard of males killing kittens, but I have witnessed many of my transient males tagging small kittens around after them and actually caring for them!  Could her kittens have died?  Would it not be normal for her to have milk, swollen teats, and fur missing if she had 2-3 week old kittens?  I'm just concerned.  I want to take her to have her spayed, especially if her kittens are gone and there is the possibility she will come in heat again,  but if she is feeding babies I don't want to whisk her off to have surgery and possibly be gone overnight leaving a bunch of helpless babies. 

Suggestions please? 

@sarthur2 had a great suggestion for this.  Using your phone, play tiny kittens crying.  the mom will run to her litter to check on them.

Oh wow, that is an awesome idea!  I'll try that this evening and see how she reacts.  I hate it if she lost her babies, but I definitely don't want her having any more.  I have a policy around my house, if they become regular lurkers they get the snip snip or the spay! 
At least my vet is inexpensive!

Is there any way you can get access to the neighbor's shed? It would be best if you could take the whole family inside.

From what you're saying, it doesn't sound like she has any living kittens at this time. I think if you can't locate the nest, spaying her now is for the best.

Thank you for all that you do! Is there any way that Tina could become an indoor cat?

@talkingpeanut   I'm afraid I can't.  Our neighbor is a crotchety old guy, and we've had several disputes over the property line.  He's very hateful and we've had to call the sheriff's department before on them in the past to have their vehicles removed from our land, so we don't speak at all.  He (and his ex-granddaughter and her kids) would probably just go out there and kill her if I told him she was out there.  They probably don't know she is out there, judging from the overgrowth around that shed.  I know due to his health he hasn't even been in that shed in probably five years, so at least it would have been a place undisturbed.

I wish I could make her an indoor cat, but sadly, we are about out of room.  I'm integrating a newbie (site swapping in progress) right now.  He was one of her BFF's--and a possible dad to her bunch of kittens.  He was a very friendly huge tom that got injured in a fight, and I took the opportunity to take him to the vet and neuter while he was there.  We brought him home with intentions of releasing him, but because he had to stay inside for a while to heal from a nasty abscess, my hubby decided he would join our house!  I also have a very elderly lady (with kitty Alzheimer's/dementia) who lives in the laundry room, because that is the only way she is calm and happy, in a small space.

I'd love to have her little sweet face in my house, but I'm honestly running out of room!  And my baby of 12 years isn't happy at all with newcomers--hence the site swapping that is now going into month 2!  She is absolutely a doll.  Solid black, little tiny face, and sweet.  Here's a few pics of that baby face.  I was having to feed her in the back end of the truck to make sure she got lots of food, then I got to where I would bring her inside to eat.