Acquired Phimosis. Action i should take.

Hello Doctor I have had a completely retractable foreskin for my entire life. But since last month, I've had this ring of tight foreskin on my penis. The foreskin goes back entirely during the flaccid state. But during an erection, the foreskin does not go back entirely. It only goes back a little and reveals some portion of the glans. There is no pain or redness and urine is normal but I'm having trouble masturbating. I first noticed this a month ago after I came back from holiday. I've been doing light stretching of the foreskin erratically but I haven't seen any noticeable improvement in the tightness and I'm starting to get a bit worried. Please throw some light on this problem and suggest the course of action I should take.