Indeed.
Get a heat-monitoring software, there are many good free ones.
Check what your stock temperatures are, play for a good ten minutes and check it again.
Also, give specs, to speed things up.
This could be a lot of things imo, I can't tell what, especially from so little detail.
I had fps-problems once, but that was a loose cooler that wandered off a bit when I carried my PC box to a friend's house.
I had no idea why the game was stuck at 2-5 fps with 20 NPC-s on the battlefield,
Turned out that the CPU cooler came off loose. It was so hot that you could probably make an omlet on it.
lol.