Fatigue whilst losing weight?

Hey guys,

I've been actively trying to lose weight for a while now (since January), but I've gotten a bit more serious about it over the last couple of weeks.

Background info: Since late last year I've had some medically ambiguous metabolic damage/thyroid issues and gained over 5kg, but I also had stopped working out consistently until january. Between january and april my weight hardly shifted at all despite starting to work out consistently and get into a routine. I never counted calories regularly because I have OCD and obviously that's probably not the best mix, but just tried to intuitively eat well, and not deprive myself. I'm also vegan.

Anyway, fast forward to a couple of weeks back when I bought a fitness blender plan (bodyweight), and decided to really clean up my diet (nothing with added sugar and no alcohol) to see if I could reverse this metabolic damage a little. So now I've been exercising 6 days a week, and when I counted my calories for a single regular day they were at 1500 (I'm 165cm / 68kg). I was feeling fine until yesterday (day 8 of the program), I found the workout reaallly hard, and slept 10.5h after it, and then today couldn't even get through a warm up.

Anyone been through anything similar? Could it be overtraining, or too much of a caloric deficit? Or both??