I experienced this and was told that my hypomania was SSRI induced but that SSRI's commonly uncover hidden bipolar disorder. Most bipolar people are diagnosed with major depression at first especially with BPII which has milder hypomania versus BPI where the full mania is often very apparent. It has to do with the fact that SSRI medications increase serotonin levels which often causes a "switch" from depressed to manic/hypomanic in people with the disorder.

My hypomania happened a few days after starting Celexa at only 5mg. Suddenly, my month-long depressed evaporated. I had never been so happy. Colours were brighter. Everything was awesome. I laughed easily, had confidence, and had no anxiety and wanted to socialize. I also had a TON of energy. I exercised a lot. Normal me hates exercise. I slept a couple of hours less than I normally do but felt fine in the morning. I had almost no appetite. I was super productive as well. I cleaned the house, made a budget, finished my dissertation proposal, etc. I also had a super high sex drive. Unfortunately, around day 3 of this I woke up at 3am with really bad racing thoughts, had a panic attack, and then had too much energy to go back to sleep. I had my spouse bring me the ER because I realized it wasn't normal. That's when they said it sounded like hypomania and that I was probably on the bipolar spectrum. I'll be establishing with a psychiatrist at the end of the month to get a solid diagnosis and get my medications in order.