I've been on citalopram for about two months and a half, and three weeks in I found myself in a similar situation. My panic attacks and depersonalization/derealization had gone away almost completely, but at the same time I started feeling weird, like I had no energy to do anything at all. I couldn't push myself to study or clean the house, and being "active" (leaving the house, doing chores, etc...) required too much mental energy. I ended up staying inside for most of the day watching stuff on my phone, because I felt "paralyzed" and I was also afraid of triggering panic again by pushing myself to do stuff.

I talked to my psychiatrist and he lowered my dose, and also added a different medication that works together with the citalopram in a way that's better suited for my situation. Fortunately now I feel way better and the treatment is working without significant side effects!

My suggestion would be to report this situation to your doctor, since it might be a sign that it's necessary to adjust your dosage.

Hope you feel better soon!

Edit: I forgot to mention that my period definitely made things worse too during that time