Human medicine for cats?

Dear Whom ever it concerns,
Hi my name is Lindsey. I got this stray cat 3-4 months ago from a barn and she is inbreed. When I got her everything seemed like she was healthy except she had a bit of a cold, like she was congested. And ever since I had her she has been sneezing a lot and that's it and snot would come out of her nose when she did. So, my friend told me to get some children medicine from the store and give her like 1/2 a tablespoon so I tried putting it in her food, she didn't touch it. So I put it in her water, she still didn't touch it. So I just gave up. Well, today I woke up and she had a hard time breathing and when she tried to sneeze nothing would come out and she still couldn't breathe. So, I looked through the house and found a sringe and I put 1/2 tbs of equate, children's allergy relief and I put the sringe in the side of her mouth and gave it to her. Two seconds later she started foaming at the mouth. Now, I don't know her weight and she never had any of her shots. So my question is this; Is it normal for a cat to foam at the mouth when giving it human medicine? Or is it something I should be concerned with?
Thank you,
Lindsey