Welcome to our forum astroscopesuk
When your GP put you on levothyroxine he should have taken a blood test at around the 6th week and given you an increase in medication.
50mcg is a starting dose and it may suit some people but they wouldn't be complaining. The problem is that many doctors haven't a clue about clinical symptoms and diagnose only by the TSH and tell the patient their TSH is now fine (if you believe in fairy tales) and you are now in the 'normal range' so medication will not be increased.
All of your symptoms are of hypothyroidism, weight gain, muscle/joint pain. and the same (except weight gain) when first on levo.
We have to learn and read as much as possible, I am afraid, if we have to get well.
The first thing to do is get a print-out from your surgery of your thyroid gland blood tests, with the ranges, as these are important as labs differ and it makes it easier for members to comment. Post on a new question if you don't have them to hand.
Make an appointment with the doctor after you had some advice, and ask for new blood tests, TSH, T4, T3, FreeT4 and Free T3 (probably wont but ask anyway). He should test for Vitamin B12, Vitamin D, iron, ferritin and folate as we are usually deficient and can cause problems in themselves.
When you go for your blood test (you should choose the earliest appointment) don't take levo until afterwards. If you take it beforehand it skews the results. After about 3 days ask for a print-out and post them too.
Despite the guidelines laid down by the British Thyroid Association, we can improve.