I have a 18 year old mare who had a problem with unrinating in small amounts I thought maybe bladder or kidney problems took her to the vet he said mares do this I took her back and asked for her to be treated for an infection so they put her on penicillin but it didn't change anything he said her urine gets stuck in a pocket coming out and that's what makes her pee continually but yesterday and today she don't eat and seems stiff in hind end she will go to vet next week but has anyone had this problem before?

No, never had a mare that had this problem. I think you need to find a different Vet AS SOON AS POSSIBLE. 

If she is and has been pooling she will most likely have a uterine infection. She needs to be cleared of that and then given a caslick (stitches to the vulva). The uterine infection has to be treated directly into the uterus. It is a procedure best done at a vet clinic. She has to be in heat during the treatment so that the cervix is open. 

A bad uterine infection can affect how the mare feels, both in her eating and her movement.

I told the first vet I thought it was a uterine infection but he said it wasn't and I took her back and asked if she could be put on penicillin because I thought that's what it was and he did give me something to give her but it didn't help we will be going to a different vet Wednesday.

With my very limited knowledge, I don't think pennicillin is the right antibiotic for urinary infections (at least for humans).  SMZ's might be a better option.  

My gelding and I are heading to the Vet also on wednesday, he got an abscess from he** in his left front hoof and that thing just wont quit, going back for a follow up to see if we are making any headway..

Edit to add, I hope this different Vet can give you better answers and help your poor mare. 

Vet checked her out today said he's never seen nothing like it her blood shows something is off in kidney so she will see another vet outside El Paso