I can’t help with the pain medication—I get nausea when I take opiates, so I didnt end up taking them beyond the first week. I also didn’t have chemo or radiotherapy, so my experience is limited in usefulness, I’m afraid.
That said I’m about your height, naturally thin and I dropped a scary amount of weight as well (about 25 pounds) though I was lucky to have my symptoms dissipate before I got too low. If I’d dropped much more they’d have put me on protein shakes. I got down to about 110, which was pretty scary—I had constant diarrhea and cramping and just couldn’t keep enough down to stay healthy.
My doctor told me to eat anything and everything I could, reguardless of “healthiness” I found that for a while plain noodles (no sauce) and plain chicken was all I could handle, so I had every variation of that for several weeks (my local Ramen place was a godsend). Baked potatoes (plain, without skin) were a good comfort food too. I also ate frightening amounts of plain ice cream (Because how many times does your doctor tell give you a free pass to indulge, really?). I avoided anything with seeds or nuts and avoided most fruit (which was the hardest thing to give up). Bananas and Avocados worked well for me, though some folks have had trouble with them. Smoked salmon was also doable. You might check out the BRAT diet (with some of the modifications they mention at the end) https://www.firstforwomen.com/posts/bra ... ers-157833
My doctor recommended against heavy exercise because of how fast I lost weight. I could walk for fitness, but no running or weight training. I’m still on those restrictions for another month or so.
You might ask about probiotics—I found that probiotics helped immensely—I used Yakult which is just a yogurt drink, but others may have more potent suggestions. If I’d been taking a probiotic regularly, I’d likely have not lost quite so much weight. I have a feeling that getting the gut bacteria back into balance is more important than we know—my surgeon actually recommends it to all her patients, and it made a huge difference. I’d ask first, though—my situation was rather different from yours...
Good luck, and I hope some of this helps.