Is rescue cat in pain?

Hello

I took in a stray female cat about 10 days ago. HER behaviour is odd. She attached herself to my screen door and started making a fuss for me to help her, so I brought her in and fed her,and cuddled her. She seemed to like that and began purring, she was happy that I helped her. I later gave her a flee and tick collar and worm medication and lots of good food. The question I want to ask you is why would a stray cat start to growl and hiss if you tryed to pick her up sometimes but not other times?,she would also become vicious, and she would do the same thing if any of my other pets passed by her. She appears to have been abused by someone, or appears to be greatly traumatized by what ever she has been through. It also appears that she may have pain in her stomach and her back end, near her tail. What could I give her for pain? I believe that human medications can be given, but you would need the right one. I can not afford a vet but I would like to help her myself. I am concerned that she may have been scarrded psychologicly and she would need a long time to heal from her trauma. Please tell me how I can help her by telling me what medication I can give her for pain or is there something else I can give her to keep her calm and not so traumatized?.