What could cause a positive iFOBT (stool blood) test in a young 23 yr old male? And is it serious? Doc referred me to GI

Many things! I'm assuming your doc ordered the test for a symptom, or anemia work up. If so, could be stomach ulcer, ulcer anywhere along gut, to bleeding diverticula in colon or even a hemmoroid. Also, vitamin with iron, nosebleed (because swallowed blood) or anything where blood gets into GI tract. Older tests would also turn positive if you had eaten red meat or spinach due to iron. Hope that helps!
True or false +'s... ...are possible. Whether blood is ingested, or originates in your gut, an occult blood test may be positive. Consider stopping any aspirin or NSAID use now (unless prescribed), and keep your GI appointment please.