I had sex a few weeks ago 1st time. i have yellowish discharge that is kinda lumpy, but liquid when aroused. do i have an std or sti? both were clean

Vaginal discharge: It's always best to have any new or changed vaginal discharge professionally evaluated. Yellow color often means infection, and at your (presumably young) age, STDs are always a potential concern. Variation with arousal probably not imporant. Since you're newly sexually active, your own physiology may be adjusting and all may be fine. But best to check it out: better safe than sorry!