That is heavy flow, but it's not likely that it was a pint of blood. (I've had some experience here
However, if this continues for much longer, definitely go to the doctor. The general guideline, at least here in the US, is that soaking through a (heavy) pad in an hour or less for a day = need to see a doc.
Also, I know you said you were careful, but don't rule out a miscarriage. Sperm can be sneaky little buggers, heh.
One thing you might consider too, even if this clears up today, is keeping a chart of your flow, notes, etc. I know it sounds kinda gross, but it's hard to remember the exact dates/flow later, and having that information can sometimes prove helpful. Include things like: how heavy the flow was, if there's any particular change in color or odor, if there are clots, the dates you have sex, what time you take your bc each day, if you have cramps/discomfort, if you have any other symptoms like headaches or blurred vision, tiredness, etc. You might also consider keeping track of what you're eating.
It's a lot of work, but it's more fun than the uber-periods.