I took a home pregnancy test and it came out negative and my monthly is 3 weeks late and that is very unusal for me

It could be that it's just too early to test. Not all women get a positive result early on. Heck, some women never get a positive result with an hpt, even when they're 8 months pregnant.

Wait until your next period is due. If you skip the next one, retest with an hpt, make sure to use first morning urine, it's more accurate.

It could also be that you're just having an off month, happens to all of us.

If you're really worried, see your doctor. :)