I had one in a pot for years in JI3 and leaf mould. But the pot was quite big. also the heat wont have helped yours.
if you only potted it this year it will still be settling in and you need to give it more time.

Seaburngirl - ahh yes the need to be 'more patient' theme again! Ha, Ha... I think the leaves going red has got me worried. Also the fact that it was gift from a dear friend makes me want it to do well even more!!

I love and have lots of peonies and I agree that you have to be patient with them. They take sometimes few years before they settle and bloom. Also peonies do not like to be planted too deep and like colder period in winter. I know some people throwing ice cubes on them in winter time.
Too much sun exposure will turn leaves red . They should be protected from midday sun...... also lack of phosphorous but because you recently planted it I do not think this is your problem.
I am not a peony expert just a lifetime owner and lots of observation.
I hope one day it will reward you with its beautiful flowers.