If it were a mood disorder, the lithium could help. (unless not at the dosage that is effective for you in particular, or it does not help you in general)
Are there any other mood disorder symptoms? It totally makes sense if there are, as it would be odd that hypersexuality stands as the sole symptom. (unless it is something else)

SSRIs (such as Zoloft) are able to trigger (hypo)mania in those with a mood disorder, particularly bipolar mood disorder.

So my guess would be that the antidepressant you were on triggered this. Your doctor should take this seriously as what you are describing can be a side effect. They will likely assess the other symptoms you have and then make a diagnosis if they see it as a mood disorder.

If the other mania symptoms go away, but the hypersexuality remains, it may be something else.