If she is pregnant then you are already doing the right thing for her. You have given her a wonderful home, good food, plenty of fresh water, and the most important thing....LOVE! If she is pregnant you will know it soon enough. Don't worry because she will be fine. You don't need to do anything extra special. If you knew for certain then I would suggest kitten chow because it has lots of extra vitamins and things to help her but she will be fine without it.
Don't worry meowmie and just keep loving her. If she is preggo then you will be able to see and feel the kittens moving before long. I always thought that was so cool!
She may be gaining some weight because she finally has a good source of food now. Some strays will over eat for a while until they figure out that the food is always going to be there.
Keep us posted and don't worry because she will take care of everything!