My cat had kittens twice in a week and a half! Is this normal?

We seem to have adopted a stray cat, we call her tootsie. She was pregnant when she started hanging around our house. She is the sweetest cat ever. We were letting her live in our garage for a while. We were feeding her and everything, made her a bed to have her babies in and everything. She had her babies. Not one of them lived. She had 4. After that we let her out again. Not out out. She was still always at our house, we just let her leave the garage when she wanted too. She decided to make our porch (closed in porch) her home. I have a cubby on the porch (because I do home daycare) and she decided to make herself a bed and lay under that. We kept feeding her and everything. She was very good and seemed healthy still after having her babies. We came home one night (no more than 2 weeks later) and I heard whining in the cubby. I took a look and there was a little kitten. The kitten is healthy and eating and so is the mommy. I have never heard of this happening ever. Is this crazy?? Any advice on what I should do????