I also can't immediately see a relationship of this condition to the dive. Faintness/dizziness only while sitting, especially prolonged sitting, is rare but not unheard of. Although one would expect it to also occur while standing, the person may (unconsciously) react early to the discomfort and employ compensating actions such as postural or leg muscle shifting which should help reduce venous pooling.
The following questions are for your consideration and will likely be asked by an examining doctor. You certainly don't have to respond to them here.
Has this happened before? Have there been any changes in your health in about the same timeframe? What about your diet? Would you call this dizziness as being "light-headed" or "foggy" or "swoon"? Or is it more like spinning? What are you doing while sitting? How long are you sitting before the dizziness occurs? How long does it persist? Does anything make it better or worse? You said it happens only while sitting but can you replicate it another way, in other positions, perhaps by also tightening your abdomen or bearing down as if straining during bowel movement?
