I was 5 days late for my period. I got it this morning and it's been bright red and very clotty and soaking through tampons every hour. What's wrong?

See your doc. Bright red blood and clots from your vagina, especially much heavier than normal, needs to be evaluated right away. An er visit would be a good idea.
Possible misscarage. As above, but if it was, its so early that you will not need a d&c, however, you should see a gyn doc.
Are you pregnant? The flow you are describing would not be considered normal, and needs treatment. Any time you are late for your period, you have to make sure you are not pregnant. The bleeding you describe could be an early miscarriage. If a pregnancy test is negative, you probably have a hormonal disturbance that will require medicine to correct. You should see a women's healthcare provider, right away.