Reggidoo

Last night my two friends and I smoked. After a few hours when we were ok to drive, we pulled over for a second in a parking lot in a park. One of my friends had to take a piss, and the other wanted to call his friend on a cell phone. We were there for maybe 10 minutes, and as we were about to leave a cop came in with his lights on. At first he was very very manipulative. Some things he said were "i followed you all the way up here you were all the over road" (which was not true, and I definetly do not understand how he could have been following us). Unfortunately I was not in full understanding of my fourth amendment rights, and we told him that there was marijuana in the car. He said something like, there isnt anything in the car I should know about is there? If there is tell me now because it'll be better for you if you tell me, rather than me finding it. Finally our parents were called, and we sat in the parking lot for probably an hour and a half. The issue right now is, if we tell the police who we bought the marijuana from, we will not be arrested. However if we do not, it appears that we will be. We have to call a detective by monday. If anyone can help me I would appreciate it very much.