I had protected sex for the first time 3 days ago and now I have back pain and strange sounds in mystomach,could I be pregnant?

Yes, it is possible you are pregnant. Any time you have sex it's possible.

However... "symptoms" 3 days after having sex is just not going to happen. Back pain and strange sounds in your stomach after 3 days? No.

So are you pregnant? Maybe. Maybe not. There is no way for anyone to know that yet. You need to miss a period and then test a few weeks after missing it.