Big ((((hugs)))). I have the same reaction to short-naps, so can totally relate.
First, a few questions: how old is your child? Has he/she ever slept for 2 hours at nap time? HOw long have you been on 1 nap? Have you noticed signs of teething or illness?
Not knowing anything else about your child, my guess would be teething or illness is on the horizon causing some OT. I think even 5h15 is probably too long if teething or illness is in the works. NOrmally, you would have to change the naptime by at least 30 minutes to fight the OT monster; 15 minutes simply won't work. 15 minutes works great when we are slowly trying to
increase awake time, but is not enough for fighting OT. 30-45 minutes is more useful.
Answer the questions and I'll see if I can't provide some more assistance. Hang in there!