Hi Jonathan, welcome to the forum 🙂 Sorry to hear of your diagnoses. We do have members here who have both, and depression is also more common amongst people with chronic conditions like diabetes. As you say, it can be very difficult to tell which comes first - the mood swings caused by high blood sugar levels or from the mixed up brain chemicals. It depends a lot on what your levels are like and if you can find an explanation for them.
For example, if your levels are high because you have been feeling down and comfort eating, or simply not having sufficient energy to manage your diabetes properly, then you may end up with high levels which will further affect your mood. If your levels are reasonable then it's more likely that it's the bipolar that is causing your mood swings.
Please ask any questions you may have - nothing is considered 'silly', and we will try our best to help you. If you're looking for friendly support from people who understand what you're going through, you'll find plenty of that too 🙂
I look forward to hearing more from you. 🙂