The body's hormone regulation system is very complicated, but for the most part its function boils down to trying to maintain homeostasis (that is, keep all the levels pretty much the same, and keep the balance among different hormones the same).
When you perturb it slightly, it'll try to compensate. Lower your testosterone just a little bit? It'll make more. Raise your estrogen just a little bit? Again, it'll make more androgens. The amount it can compensate is limited by the availability of certain key chemicals and by the number of receptors for various hormones used to send signals. It can compensate for small changes pretty effectively; larger changes can overwhelm it and force it to adjust to a new homeostasis. If the doses you're taking aren't large enough to overwhelm your body's natural regulation system, they're not going to have the intended effect, and in fact may have the opposite effect.
Add to that the fact that your brain can respond rather oddly to changes in your neurochemistry. It's not always predictable. For some women, birth control pills stabilize mood; for others, they precipitate debilitating depression. It's pretty much a given that a hormone change is likely to trigger a mood change, but exactly what that change will be is unpredictable.
In other words, what you're experiencing now isn't necessarily what you can expect from HRT in the long term. But you probably should get with your doctor and explain your concerns and have your levels checked.
