Can u still be pregnant and have a light period?

Kinda sorta: It's a matter of semantics. No, you won't have a normal, regular menstrual bleed. But yes, you can bleed/spot while pregnant. Check out http://www.womens-health.co.uk/menstruation-during-pregnancy.html and report your concerns to whoever is providing your prenatal care, Family Doc or OB.

Isn't common: on the other hand there can be bleeding for many other reasons than a menstrual cycle. Implantation they tell me can cause some spotting. Certainly irritaiton/allergy/infection can cause some spotting.

Yes you may bleed: Early in pregnancy, you may experience (Not a light period) but some vaginal spotting, staining or bleeding due to "implantation of embryo" into your uterus, early threat of a miscarriage, "decidual reaction of an ectopic pregnancy, or other unknown causes of placentation problems..Bleeding may also be caused by changes, inflammation or infection in your cervical tissue.