Should i take this medication?

[Amy214]

I have suffered with back pain on and off since i was pregnant with dd (she's 2 years old now) a doctor told me it was because i was overweight since then i have lost 2 stone and it has gotten much better. When i was on my period i used to get back and pelvis pain but since losing weight and skipping exercise during my period the pain has gotten much better over the past 7 months. Yesterday i exercised (just before my period started, i know when it starts as im on the combined pill) everything was ok during the day but when i lay in bed at night my back was very sore (lower back down to coccyx) after painkillers and a hot water bottle the pain subsided enough for me to sleep, when i woke this morning the pain was gone but it would flare up every now and then. I made an appointment at the doctors and he prescribed me some amitriptyline now i've read some reviews online and most people have said they gained weight (i don't want to gain all the weight i've worked hard to lose!) my doctor did tell me i am still slightly overweight so i want to lose the rest and he recommended walking and swimming. I know i caused the pain by exercising because im on my period, so i don't really want to take the tablets unless it hasn't gotten any better. When i exercise any other time of the month my back feels fine during it and after it. Can anyone help?