Merry Christmas everyone!!!

I took my bike out today. I'd been out for about an hour, was accelerating really fast, but when I changed up from second to third there was a very delayed response, like maybe the clutch wasn't engaging properly. Anybody got any other ideas what may be causing this?
The bike's a 2000 J1 with about 13,000 miles on the clock. I had put a Remus Genesis can on it back in October, and haven't had the bike tuned since then so I wondered if that may have something to do with it.
If it is the clutch, is it something I could try to sort out myself or should I get it to a dealer?