First of all, even if you don't take the medication, be sure to follow a good mental health professional, and check any advice here with that professional
I don't know if the following is what is happening with you, but it's my guess. Examine if it has any worth.
When you are applying mindfulness, you can be doing it with an unwholesome emotion at the same time. For example, if you are very angry, the mindfulness of that anger is "coloured" by that same anger _ like pure water having a red colour due to some red paint. And it can have a perpetuating effect on the anger.
What seems to be happening here is something similar to what happens to many people. And, if I recall correctly, this is explicitly warned against on the retreat. I think you are practicing the equanimity and, at the same time, having aversion to sensations. That is, since you know equanimity protects you from suffering, you are using "equanimity" together with aversion to suffering. It results in repressing emotions that are difficult to deal with.
The way I deal with it may not be a great way. I hope someone more experienced can comment on this. But what I do is generate desire to feel the suffering underlying
the unpleasant sensation. And it has to be dealt with in waves. I bring the underlying suffering to the surface and then, after a few seconds, I apply equanimous mindfulness. Sometimes, focusing narrowly on the sensations is not the best strategy. Sometimes you see the mental attachment to an idea, or situation, or person, and you just let go of it, like relaxing a muscle. Another good thing to do is to fix the "defective" equanimity. Try to find a genuine interest in feeling the sensations with objectivity. The motivation being that, not only it is healthier on a psychological level, but also that it is closer to the dhamma.
Don't forget metta bhavana! It softens the impact of difficult emotions surfacing. Some emotions are so difficult that, in my experience, it is best just to do metta for a few days and then go back to mindfulness.
And the Blessed One addressed the bhikkhus, saying: "Behold now, bhikkhus, I exhort you: All compounded things are subject to vanish. Strive with earnestness!"