Neutered cat suddenly showing sexual behaviors?

Ok, my cat is a pervert and I don't know what to do about it - I must admit I'm almost embarrassed to call my vet and ask her if this could be a sign of a medical problem or if I just have a nasty kitty HAHAHA, Osiris is just acting really strange for about the last 2 weeks now. The last 3-4 days I've noticed him "making muffins" on blankets and acting a little strange while he was doing it, didn't figure it out right away, but saw him doing it and looked a little closer - His penis was sticking out like he was trying to mate with the blanket! Then I saw him trying to mount my IG puppy, Sara last night? I may joke that Sara's more Sphynx then IG, but that was taking the joke a little to far HAHAHA NASTY CAT! When I think about it I realize he's done it to my leg as well a couple times and it just never clicked until now what he was doing. But on a more serious note, what could be causing this sudden sexual behavior? Osiris has been neutered for a couple years at least now (not sure exactly when he was neutered - He was neutered when I rescued him and had been for a while, he's been with me a year and a half). Could the pooping outside the box be marking his territory and not actual accidents like I was assuming? Now I'm wondering if some of the pee spots I have to keep cleaning up may be partially him and not only Casper like I was assuming (Casper has been naughty since I got him about peeing on blankets and stuff - He had a nasty UTI that had been left untreated for a long time, and it's now become a habit, he's gotten better but usually if I find cat pee somewhere,I blame Casper!), which is always where I find the pee spots, but now I'm wondering.....). There are no intact cats in my house, I've had them all fixed. I may just be being a bit paranoid, but especially with him having been so sick just a few months ago that we never figured out why really, it always makes me uneasy when he starts acting different! Should I take him back to the vet, or just see if this behavior continues? He was just at the vet about 2 weeks ago and we ran basic blood work (he got really sick really fast about 3 months ago now and we just re-ran his full CBC/chem panel to make sure things where still ok). Nothing was abnormal on his last blood work but we weren't testing for anything specific.

Should mention we also did a urinalysis (Normal, no crystals, PH and dilution normal, no bacteria or blood).

Thank you so much everyone for the great help and advice! After talking to my vet about him she decided she wanted me to bring him back today and re-do his urinalysis just to be sure that something hadn't changed, low and behold, he's now got a UTI! So I'm sure that's what triggered that behavior, started him back on antibiotics and we'll re-run it again in 2 weeks when the antibiotics are gone and make sure. If he's showing this behavior after that point, then we'll know it's safe to treat it purely as a behavioral problem. Thank you to those who encouraged me to double check with the vet and make sure before I went ahead assuming it was a behavioral problem, my cat does have some kidney damage caused by an infection we didn't catch fast enough - So I'm always pretty cautious about stuff like that! I'd rather have it done and find out it's nothing then assume it's nothing and end up with a really sick cat again!