Very light bleeding on period and i'm not pregnant. i usually bleed a lot so this seems abnormal. what could it be?

Many things: Sometimes stress can cause a change in your cycles. Increasing exercise, change of diet. Also, i see that you are 42, so your hormones may be starting to fluctuate more. If your periods remain on time, generally, getting lighter is not a danger sign. If you are trying to get pregnant, you need to see a fertility doctor right away.

Many causes: A lighter than usual period may result from physical or emotional stress, or an abrupt and significant increase in exercise or a dietary change. Although you are young for this one, you might be entering perimenopause, although this is premature to judge based on 1 period. May be a variant of normal, may indicate another endocrine problem - eg thyroid. Something to track and report to your doc.