What is causing my hamstring tendon injury and reinjury on both legs?!

Back in January I woke up with pain behind my knee on the inside. After a week I began to get a similar pain in the exact same place but on my other leg, just nowhere near as severe. After 2 weeks of it not improving I went to see my GP who suggested that it may be a hamstring tendon sprain and referred me to physio. The physio agreed that it was likely to be a hamstring tendon injury and set me some exercises (squats, hamstring stretch and hamstring curl). She suggested that the issue could be an imbalance between weak glutes and stronger quads. After a few weeks of the exercises I was back to walking normally and without pain. At this stage I stopped the exercises which in retrospect probably wasn't a great idea. I slowly went back to exercising again.

At the time I had no clear idea what caused it. The fact that I woke up with it obviously suggests that it was down to overuse. I run occasionally and I do resistance training but only with low weights or body weight. A couple of days before I woke up with the pain I had been swimming for the first time in years and years. I was swimming breast stroke and I know I over did it so I wondered if this could have been a big part of the problem? I remember feeling sore behind my knee on the inside at the time. I think I was kicking way to hard to make up for a fatiguing upper body and possibly damaging the tendon towards the end of the kick? I remember experiencing knee ache before the tendon pain began but I can't remember how long I experienced that for beforehand.

In May I woke up with the pain again but this time only minor. It seemed to clear up after a few days. This time it was my left leg. I remember the first injury being on my right leg, but my doctor's notes say left. I could be wrong, but I'm fairly certain that the first injury was the right leg. Then, in June, I started to get knee ache (just like before) in my left knee. By the end of June I was walking and felt a twinge in my left hamstring tendon. I knew it had gone again.

Since then, I have accidentally worsened it twice during recovery. It really didn't seem to want to heal. Finally, after two and a half months I seem to be moving in the right direction with the help of a physio but the time it's taken means I'm also dealing with a lot of muscle wasting and knee pain after a long period of limited movement. I'm hoping I should be back to normal again soon(ish).

My physio this time around hasn't been very helpful at suggesting what could have caused this or how to prevent it. I'm really worried because this has now happened twice and both times it impacts both legs. I really don't understand. Is it possible that this is a reinjury because the first one didn't get chance to fully heal? Or am I doing something really wrong somewhere and managed to sprain my hamstring tendons twice totally independantly?