Is my girlfriend pregnant? Please answer?

We've had alot of sex lately, but lately we have been using condoms and they havent ripped or came off or anything. But before, we did some experimenting. We didn't have SEX without a condom, but I stuck it in there for about a half a second then took it out, and there was no PRECUM on it. But recently she has been sick , tired, and has had headaches AT NIGHT. She isn't tired that much but usually at night a little earlier than usual. The sick and headache thing has been for a few nights in a row. We are 15, we CANT buy pregnancy tests. What should i do?!?