Ongoing bicep pain for past 2 years

Hi all. I am a 30 year old male and would describe myself in relatively good health apart from the below injury that I will explain to you all. I hope someone out there will be able to help me as I am really at a loss on what it could be!
　
To give you a brief background on my problem: At the age of 18 I first started going to the gym around 3-4 times a week to lift weights and for the next 9 years went on and off at various times during my life, sometimes cancelling my membership due to lack of motivation for up to a year at a time. In 2010 and at the age of 27, I decided that I really wanted to take my training serious for once so I focused on going to the gym 5 times a week at an hour each time. I would always start with a 10 minute run to warm up however my objective was to add muscle so I was mainly going to lift weights rather than do cardio. (I should say that I am 6ft'1 and only weigh 155 pounds and have been this weight for pretty much the last 12 years, I have real trouble putting on weight so this also put me off doing much cardio!)
　
After 9 months of my training routine all was going well and I was starting to see some real results. I was taking protein shakes to go along with my training schedule and I was really starting to gain some decent muscle. One day however I was in the gym and doing my usual bicep curls and I felt a twinge in my left bicep that caused a nasty sharp pain. I stopped training straight away and went home as I did not want to cause any further damage. Next morning I woke and I could still feel the pain so I decided to lay off the weights for a week to give the injury time to heal. I would describe the pain as 8 out of 10 in severity.
　
A week later I went back to the gym as the bicep at rest was causing me no pain whatsoever. I started with a few lighter weights and all felt ok. It was only once I went back to a higher weight that the pain came back. I was gutted as I realised there was something obviously seriously wrong and that I'd have to stop training for a while. I decided on a 3 month break from the gym to allow the injury to heal.
　
After 3 months again all felt good and I was confident the injury was healed so I went back to the gym, lifted some weights and you guessed it, the pain started again! I decided that I had to seek medical help and went to see my GP. He referred me to a physio who was unable to diagnose what the problem was. This left me greatly frustrated. I also tried a registered massues who deals with sports injuries but she was also unable to identify the problem. With regret I decided that my gym days would have to be put on hold for a real long time to give the injury long term time to heal.
　
I left it 18 months and all the muscle that I had acquired has now gone. It's been very depressing for me as I worked hard for that and its knocked my self confidence as I feel really skinny as I am now. I wanted to get back into the gym this month in time for summer but I obviously was wary of the bicep injury so i bought myself some cheap dumbells to use at home first, before committing to a membership. Sure enough after using them for around 20 minutes the pain returned! :-(
　
I honestly don't know where to turn to as nobody I speak to seems to know what is causing my pain. As i have said, at rest i get no pain at all and it is only when the bicep is in a bicep curl flex position that the pain starts. Any advice on what this could be would be much appreciated as I really would like to get healed and one day back into the gym.
　
Kind regards