If you want to take a pregnancy test and have to do it in a way that is not obvious, you might have to do it at a time of day when you have eaten and drunk, and peed. So save some first-morning urine in a jar, so you will have the concentrated urine you need for an accurate answer. Otherwise the test might read negative simply because your urine is too diluted (and the negative might not be right).
If you are pregnant, sweetheart, you really do need to have the talk with your mom, sooner rather than later. If you simply can't talk to her because the situation is too extreme, talk to another female relative.
But there is nothing wrong with finding out first. The pregnancy tests at the Dollar and Dollar General stores are really inexpensive and register accurately pretty early (around the time of a missed period). I hope you are not, I hope you were not forced, and I really hope you insist on protection from now on.
Having given birth, I can say that yes---labour IS the most painful thing I've ever gone through. If you want to find out if you're pregnant or not and don't want her to know, buy it when you're alone and then take it at a time when she's not in the house or around you. However, it would be a smart idea to mention it to her, as if you're pregnant and still living in her house she WILL find out---and you're going to need her support anyways.