Why has my dog's sheath suddenly started rolling in on itself?

I have a 3-1/2 year old male Boston terrier who I have had for 2 years. I am not sure at what age he was neutered, but he HAS BEEN NEUTERED (caps lock for all who scan this and then yell at me about the need to neuter my pet) as long as I have had him. Occasionally he gets s... this usually arises when he is trying to relieve an itch on his inner thighs, but sometime he just gets a little fresh with my plush blanket. This is nothing new and has happened periodically for as long as I've had him. Recently, however, as in, in the past week or so, he has had trouble retracting his back into the sheath. The first time, I didn't realize what was going on, and was quite scared as he seemed to be struggling to get it back in (I could actually see the retractor muscles in his heiney contracting and relaxing). Though I wasn't really aware what I was doing, I managed to pull back the skin on his underside and get it back in. It happened again today, and, after doing my homework the last time, I quickly realized what was wrong and, again, pulled back the skin to help him unroll the sheath, which had rolled inward when he tried to retract. All of this is not a big deal, as he only exhibits this behavior when I am home, and, both times, I have reacted quickly enough (within a couple of minutes) to help him get everything adjusted and back in place. My question is this: does anyone know why this would suddenly start happening when we have never had this problem before? I've checked for errant hairs that may be catching his foreskin and causing it to roll in, but I don't seem to see anything. He was recently on anti-inflammatory pills for a pulled muscle in his knee, but has been off of them for over a week. Is this maybe just an adjustment to being off of the prescription (he was only taking them for 6 days under a veterinarian's care)? Any suggestions are appreciated.