Should pain

Hey, I have a weird shoulder problem. I think it’s my bicep tendon, I’m getting it checked out soon, but anyways. On flat bench press, I feel more strain/pain on my shoulder if I use low weight/high reps, then if I use high weight/low reps. What is up with that? Wouldn’t an increase in weight put more stress on my shoulder? Also, back is the opposite, the more weight I use the more I feel the strain (Shoulder to the top of the bicep area). Could I continue to pack on the weight on bench press?