27 weeks Modified Bedrest Pains

So I’ve been on modified bedrest for 5 weeks now due to preterm labor signs. I was pretty fit going into this twin pregnancy, but obviously I have lost a lot of muscle since being fairly inactive for the last 5 weeks. Anyway, I was having occasional contractions throughout the day but my doctor put me on some medication to decrease it with the hopes that I can get up more and maybe go back to work (I’m a teacher and will run out of FMLA before the twins even arrive). The number of contractions I have are significantly lower which is great, but when I sit upright or try to test my body for a little bit longer to see if I can go back to work, I get the WORST pains in my upper abdomen/uterus. It’s so unbearable and The only way to relieve the pain is to lay on my side. It’s making me think that there is no way I can be upright for 8+ hrs a day even if my contractions are less 😩. I feel like it might be that my muscles are just so week they strain too much with the weight, and those muscles are cramping up... but I’m not sure if it’s something else. Anyone else have this awful discomfort? Am I just being a pansy and need to recondition my muscles after not using them for 5 weeks?