I've felt extra tired & moody lately...

I usually have a very regular 28 day cycle. This time around it was only 26 days. My period normally lasts 7 days, but this month it only lasted about 3. It was pretty heavy the second day, but after that there was pretty much nothing on the the 3rd day. This has never happened to me before, so I'm not sure if it's just a random month or if there's something going on. My husband & I are kind of trying to get pregnant (we're not preventing in any way), so is it possible I'm pregnant? I've heard many women mistake implantation bleeding for their period. I've taken a home test and it was negative, but I've felt extra tired & moody lately (which is very unusual for me). I have my annual ob appointment next week, so I'll be able to talk to the doctor then, but I'm a little anxious to know what's going on. Any one had anything like this happen to them or have any thoughts/advice?