Ok, first thing is first. You say you've been playing soccer all of your life and trained in muay thai, chances (and correct me if I am wrong) you are kicking with the same leg for muay thai and soccer, if thats the case you may thrown off your hip alignment which might cause the problem. Simple way to find out is to go to a Chiro and have it measured, usually from crest to toe. Even if it is off a 1/2 " it would cause problems. Now, this is where that kinetic chain that I am always preaching about, ex; If you have a knee problem on the right side and it isn't corrected it will go to the hip, the low back and so on. If you do have an imbalance in your hip it can usually be corrected with a couple of sessions of adjustments, if you get that realigned (assuming that is the problem) you will find the knee starting to trac more efficiently thus cutting down on potential injury.
yes I kick with my left and my balance is much better with my right leg.
I haven't trained in Muay Thai in over a year. Although with MT you have to be good with both legs, mostly I'm a southpaw. so I can see my balance being shifted by always kicking with my left.
I'll make an appointment with a Chiro soon. thanks for the advice.
any other exercises I can do in the mean time?