Where are all the cats coming from?

I started caring for some cats that I saw occasionally crossing through the woods behind my house.  Two tom cats and a female whom I think has been spayed since they show absolutely no interest in her.  I have limited funds because I am suffering a work injury and out of work.  My husband built some winter shelters for them and the toms have been using them on a regular basis.  The female has adopted us and stays indoors when it is cold.  I have begun researching TNR and am planning the capture of the males this coming weekend.  (A little late, I know...)  Our local Humane Society does ferals and strays on Saturday and Sunday only.  Their fee is 50.00 per cat to neuter and rabies vaccinate.

Over a month ago I noticed a cat that I hadn't seen before running down the street in a snow storm.  As I watched I saw it pause in front of an empty house and duck into the backyard.  A few minutes later I saw one of the toms stalking in the front of the same house.  Thought to myself....oh no...its a female in heat.  A couple of weeks ago she started showing up at my front porch and of course, I fed her.  Once I started feeding her on the front porch a calico began showing up as well and from the reading I've been able to do I realize she must be a female too.  I haven't noticed the toms showing interest in her, either.

Yesterday I saw another cat, a tortie, eating some of the kibble on the front porch I had left for the ladies.  Where are they coming from?  Are they all females in heat that have come for the toms?  Help!!

First, thank you for caring.
They are coming for the food. They are coming because their former owner(s) did not spay/neuter their cat(s).
There is an unwritten rule that for every feral/stray/dumped cat you see, there are 10 cats you don't see. Irresponsible (IMHO) owners of pets do not spay/neuter their cats, who then proceed to make more kittens. The mother cat and kittens are sometimes dumped too (I am the luckily recipient of just such a kitten).
There is a well known rule that "if you feed, do not let breed". GOOD FOR YOU for getting the cats spayed/neutered. If you have to chose (and, if you can tell!) I would get the females done first. So you don't end up feeding their kittens. Which is how *I* got started in TNRing (35 cats in 2.5 years).
You can generally borrow traps from low cost s/n centers and the local animal control department. Good luck, and let us know how it is going.

Thank, Ritz...I have one trap.  As far as going for the females...I have limited funds available for spay/neutering.

The males I feel responsible for because I've been feeding and sheltering for awhile but I just can't afford to do all the neighborhood cats at one time.  One of the females is well over a month pregnant if she's pregnant...the other female may already be spayed because I have not seen any mating activity.  Its very difficult to juggle what the proper thing to do is with limited funds.

Somebody has owned these cats.  They are all friendly and allow petting.  Maybe I need to ignore and stop feeding?

That's a hard decision to make. Who have you reached out to for financial help/trapping help with these cats? Where do you live (county/state)?
I have to (want to) believe there is a very low cost/free spay neuter clinic in your area. I know some county governments give free vouchers to pet owners to get their cats/dogs spayed/neutered . Also, check the websites of low cost/non profit s/n clinics. They can apply for (and receive) grants from various companies (PetSmart, Petco) that will go a long way towards paying for the cost.

I have been in contact with the Humane Society where they do ferals on Saturday and Sundays for 47.00 per cat.  I have one trap so my plan is to get the males done this weekend...one on Saturday and the other on Sunday.

I have also been in contact with Friends of Felines and they just obtained a grant from Petsmart Charities but they are busy working on targeted TNR in specific low income areas of the county.  She did not offer me any advice on financial aid and seemed overworked with her current projects.  I am in Wichita, KS.

The female that I believe to be pregnant will allow me to pet her and seems to enjoy human contact but she will not enter my house when invited.  I think she is due to give birth in about three weeks and I would like for her to come inside for her safety.  Any ideas on how to convince her that it's a good idea?  I have a geriatric resident that keeps to a bedroom most of the day and I have another bedroom I could make into a cat nursery for the mother to be.  There is another spayed female stray that is staying with us while its cold outside...not quite sure if that is why the pregnant one is hesitant to come inside or not.

All suggestions for my situation are very much appreciated!