I will sleep 12+ hours if uninterrupted

About me: 26F, 5'5" and 115 lbs taking 300 mg Lamotrigine, 20 mg Buspar, 150 mg Wellbutrin, and occasionally 5 mg melatonin. Diagnosed BPD, PTSD, and psoriasis, no other health issues to my knowledge. I drink a few times a week, I don't consume marijuana or other rec drugs.

Current PhD student but not egregiously overworked or anything. I eat 2 meals a day (not in a calorie deficit), pretty low exercise (mostly walking) and typically have two cups of coffee--one in the morning ~8 am and one after lunch around 1. I sleep 8 hours a night and seriously struggle with anything less than that (I won't fall asleep throughout the day, but I will struggle with focus immensely).

On weekends, if I don't set an alarm, I will sleep for 12+ hours with my husband usually waking me by the time it hits noon if I'm not awake yet. We joke that it's my weekly coma. My body does not wake up without an alarm.

I kind of thought this was just catching up on sleep from the week (although I'm not in a sleep deficit), but taking two weeks off for winter break, I am still able to sleep 12-14 hours every day. I feel well rested after this, but again, if I don't set an alarm, I just won't wake up, and I'm wasting half a day. Does anyone have advice or ideas what could be going on?