My opinion - by adding the mood stabiliser, your base line is lifted. The anti depressant effects are now extenuated and pushing you into hypomania.
Remember, for BP the first line of treatment is a mood stabiliser. Anti-depressants should be used cautiously due the risk of them making you manic or hypomanic.
I've been on 200mg of Lamotrigine (Taken all at night) and will now be adding 50mg for the morning. With an anti-depressant on top. I think I'm becoming a bit too resistant