Yes keeping a dairy and journaling between sessions is a good way if tracking your moods. I think it's normal when we are going through something and attending therapy to feel this high and low feelings and to go forward and take a step back. It's also normal to want to blame something or put a diagnoses on our feelings or label them as something in order to explain or accept that we are feeling down and that's it's ok to feel down. As long as our moods don't interefere with our work or relationships.
Yes this is a symptom of bipolar but it would have to be extreme mood swings along with other symptoms.
Do you feel like these mood swings are uncontrollable or can you pin point them to a trigger?