There should be a Planned Parenthood Clinic near you. Please do not wait much longer for a prenatal check-up..........I respect your belief that your baby is from God, but it's the real life doctor who is going to make sure both you the baby are in the best of health.
Why not tell your mother now and let her start getting use to the idea that you ARE going to have this baby. She cannot force you to abort and even if she could (or would) a second trimester abortion is, to the best of my knowledge, against the law. Besides, you really don't KNOW how she is going to react. I'm guessing there may be some anger and disappointment at first, but she may turn out to be your strongest ally.
Please remember, you believe you are mature enough to have a baby, in the coming months you are going to have to prove that over and over.
Peace
Greenlydia
There are health clinics all over the US that you can go to. Maybe try your local health department, http://www.idph.state.il.us/local/home.htm, which is usually low cost or free. (I saw that you live in IL). There you can get pregnancy testing to confirm you are pregnant and get prenatal care.