Do you have a weaker leg/ankle? Without noticing, you could be putting more weight on one foot than the other. Make sure your stirrups are even. I have an something that could help.
When you mount, before walking, stand up in your stirrups. Get a spectator to tell you whether you stirrups are even, and also whether you are standing straight or lopsided.
Walk around in this fashion, weight evenly disputed on either foot. (The first few times you will collapse. I used to do this every lesson and now I can do it for quite long without sitting. ) When you feel comfortable, check your stirrups again and trot. Trot over a few poles like this as well.
Don't know whether that will help or not.
If your horse says no, you either asked the wrong question, or asked the question wrong.* ~Pat Parelli