I trimmed my bougainvilleas in early February, nothing too harsh, perhaps about 1/3 of the way back. There was an initial leaf drop (pretty minimal), as expected when trimming the plant, but since then (now mid-April) it has been steadily losing older leaves (Yellowing, followed by drop). Most recently, I've been finding odd spots on the leaves, as you can see in the picture. Most of them are on the yellowing leaves, but a few green leaves have them as well.

The plant seems to be blooming just fine, and there is some new growth starting to come out.

What should I do?

I've read that the plant needs magnesium, which could be the issue with the yellowing leaves, but I'm not sure what's causing the spots. Could they be spider mites? If so, what's the best method of treatment?

Any advice is greatly appreciated!