Shoulder pain in my right shoulder?

Okay about a 4 months ago i was in the pengers seat of the car and reached back to get my backpack which had about 5 books and was really heavy. about 30 minutes later i started experiencing pain in my shoulder like a sharp pain when i would abduct my arm to about a 20 degree angle pain was towards the front. i had that for about a week and then it went away. now about 3 months later it came back yesterday. the pain is not very bad i would rank it about a 3 out of 10 with 10 being the worst. but it is kind of a nuisance and annoying it feels better today but i was wondering if i could lift weights today. or if i should go see a doctor or even worry about it