Of the drugs that you are on, Gabapentin can cause loss of appetite while buprenorphine and naproxen can cause constipation. On the other hand naproxen can cause weight gain. So, if you are experience constipation and loss of appetite, then yes, these drugs could be responsible for the weight loss. Otherwise weight loss could be due to not eating well, or due to hyperthyroidism, diabetes or worms in gut etc. The symptoms you are now feeling could be due to buprenorphine withdrawal. So, please do not change, start or stop pain medication without consulting your doctor.
Apart from pain medications, it is always advisable to try a non-drug therapy such as physiotherapy, yoga, meditation, or hydrotherapy. Please discuss with your doctor. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.