I've also always had heavier periods. This month I was 15 days late, but when I started this month it was lighter. My fiancé and I are trying to have a baby but the test said neg.
You can't get pregnant a couple fo days before the start of your period so that;s not an issue. If you've had sex before than, then maybe you could be.
Sometimes, our cycles just change for no reason, though. Stress, hormone fluctuations, diet/weight changes can all affect your cycle. A major one that can also affect is a new exercise regime - which you've been doing. So if you take a test and it's negative then it's most like the new exercising is the cause. It's not a bad thing - it's good having lighter periods!
Most likely you are not pregnant, but always be sure to use some form of protection when having sex unless you are wanting to conceive. Implantation bleeding sounds like it's far lighter than what you're describing from everything I've read.
It could be you're just having a lighter period this month due to a hormone change or it could (and I would lean toward this being the more likely explanation) be all the exercise because exercise is known to affect your flow. Some female athletes don't even end up with their periods because of all the training and the like. Just relax and enjoy the change.
I know what it's like to have heavy periods, by the way. You might look into birth control as it will lighten your flow and make periods far more bearable. I went from a very similar flow you describe as your normal (7 days, heavy with clots, unable to function because of that and nausea, fatigue, etc.) to 2-3 days light to moderate flow. Besides, if you're having sex, it wouldn't hurt.