I've Been On The Depo For Almost A Year And

I've been on the depo for almost a year and a half. The last time I got my shot I spotted near the time my next shot was due. I am due for my next shot in about 16 days and I've had a really light bleeding for 5 days now that has been bright red but its enough to use a panty liner/tampon. Is this normal being on the shot? I've been on the shot before for 4 years without one period, and then got off for a year, got pregnant, and got back on the depo at my 6 week checkup after pregnancy. I've been on it for a year and a half now.