Greetings.
The discharge you describe is unlikely to be related to STI. Some men may have whitish or greyish discharge of seminal fluids in the morning, that is normal.
Stomach ache is unrelated, STIs never produce any sort of stomach pain or cramps.
But since you engage in unprotected sex (as I understood, with multiple partners, sorry if I got it wrong), you are at risk of every possible STIs (including HIV), and some of them are asymptomatic. You should be aware of the risks and get tested regularly.