Anyone experienced implantation bleeding on/around day 21 before? How long did it last and is there any cause for concern? I'm having light bleeding which is bright red but not heavy enough to be period. Am very worried.

Implantation bleeding can happen up to 10days after unprotected sex but it usually only last a day and spotting kind only. If day 2 still have, not likely it's implantation bleeding. The fact is that there is nothing at all that u can do unless u confirm u are pregnant then can take medicine from Gyne or jabs but u are not officially pregnant yet. Sometimes there can be mid cycle or off timing bleeding due to hormonal changes - us human body can fluctuate quite a bit, stress, foods, all can affect the hormones - when there is not a smooth enough changes in hormones, some bleeding might occur. If it happens for a few times already and not pregnant - do schedule a Gyne check for other stuff