Yeah, the muscles in that area clench when orgasm happens (primarily the vaginal muscles, but other nearby muscle groups can as well).
I guess you could try putting some lube in and around your ass to see if that makes the clenching more comfortable and thus less likely to leave lasting discomfort after your orgasm.
Also, if you do Kegels regularly, maybe you should stop, as it seems your muscles are plenty strong enough already!
See your doctor if you’re concerned about it and want answers from someone more informed.