i usually have periods that are heavy and last 4 to 6 days. This month my period came on the same date as usual but lasted less than 24 hours and it was reddish brown. What could be the problem?

It's common and normal for women to have fluctuating periods, especially during the teens and early 20's, with variances in the duration, flow, and other factors. Most likely there's nothing to worry about, but if you experience irregular periods for several months you should talk to your gynecologist.