I am so annoyed! my index finger above the top joint only, is swollen and firm! a bit red. i don't remember injuring it at all. could it be serious?

Not serious: In a 19 year old, one possibility is a paronychia which is an infection around the nail area. There are, of course, other possibilities. Have a doctor take a look.

Possibly: There are numerous causes of isolated non-traumatic joint swelling in the finger. Proper diagnosis of the cause(s) depending on examine your diet (gout), risk factors (history of IV drug usage with endocarditis and splinter hemorrhage, co-morbid medical conditions including diabetes, inflammatory bowel disease, psoriasis, infections), and presence of autoimmune rheumatologic conditions.