Geriatric Kitty visits litter box, does not do anything

I adopted my 16 yr old Lily 4.5 months ago. She was 6 lbs had fleas, worms and had not seen vet since 2011. She was diagnosed with hyper-t and crf, put on prescription k/d and methimazole. After a few weeks she adjusted well and now weighs just over 8 lbs. Generally good appetite, plays with toys, very affectionate. It has not been easy but so rewarding. She also gets famotidine once/day and a packet of fotiflora divided among several servings of wet food.

Yesterday we saw vet. End of Dec. Lily's red blood cells were a bit low so she got iron and B12 shot. Vet gave her another iron shot yesterday (I gave B12 shot one week ago, which she gets once a month). Came home and she was so exhausted from vet visit I skipped the famotidine since I didn't want to bother her more. She ate well last night and today. Pooped 2x this AM.
But now for the past few hours she keeps visiting litter box, paws around in it as if prpping to poop, then leaves without pooping or peeing, roams the room, meows repeatedly, goes back to box, repeat.

She has not needed subq fluids as yet but can anyone explaun this behavior and could it be caused by iron shot? She usually has soft but well formed stool.Vet prescribed metronidazole for that but I have not started it yet. Waiting for blood and urine tests results. Thank you for reading. She is a lovely animal and I am so lucky she found ne!

First thing is I am so grateful for people like you that adopt a cat of senior age, they do the worst in shelters, confused about what happened, why they are there, and the noise and smells, it's all very overwhelming for any cat but even more so for a senior.

Iron can possibly cause constipation, not sure it would happen so fast though.

More than anything, watch to be sure she pees.

I see this thread was posted a couple of days ago, how is she doing now?

Thank you for responding! She's doing better, and yes, you're right about monitoring her urine output.

I actually rescued her from her previous owner. When I saw her condition I begged him to take her to vet. 3 weeks later he did but when he learned the cost of tests for likely crf, etc he took me up on my offer to adopt. I couldnt stand to see such a sweet creature suffer. His biggest concern? That she might have trouble adjusting to living indoors. I've assured him she loves being inside!

What  a darling little muffin bunny girl!!

What could there possibly be not to adore here?!  

So good to hear she is doing better!