What you're calling clots is actually passing the tissue shed from your Urterus. It's very common for women to do that. That's what our period is for. You pass it because you didn't become pregnant. I've done a lot of reading and found this out. Why do some pass a lot and some little? I can't find that yet. It could be based on how fertile or less fertile you are. I'm not sexually active, so I don't know if this comes into play. I haven't read that it matters. Many mistake urterus tissue for clots. If it was a clot, it wouldn't have the texture it has. The tissue you pass has only a pinkish/grayish color, but the menstruation fluid (not blood) colors the tissue. I also get cysts, and that tissue has a similar texture to the urterus tissue.
I suggest that if you have that happen again, try using Instead cups or Diva Cups. Diva Cups are re-usable with just a bit of soap and water. Technically Instead cups are re-usable by dumping in a similar manner. The beauty of using cups VS tampons is that it collects. You'll get to see the full size of the tissue in this manner. If you think of just how deep your urterus is, you'll understand why there's so much tissue. I would know because I pass a LOT of tissue, and my periods last at least 4-5 days, 2-3 of which are very heavy.
Considering your age (don't take offense, please!) I am thinking your period could be fluctuating because most women your age are near menopause. Then again, it is normal for women of any adult age to have their periods fluctuate often. It's quite a fickle thing, really.
More on Diva Cups. You don't have to buy them for at least a couple years, I believe. I do need to go read that again. There are different sizes for women pre-natal and post-natal. Everyone has a different method of disposing the contents and washing it, but I suggest a cup of very warm water over the toilet, pouring whilst dumping, and then washing it. There will be rogue flows of course, so maybe a bit of toilet paper to hold it back will help. I suggest re-filling the cup of water and washing yourself before putting it back in, then after, to keep yourself fully sanitary. As far as I know, these don't leak unless it didn't open all the way.
I'm sorry for the long reply, but I do hope any info I've given does help. I suggest reading Wikipedia (they DO have experts come in on the articles) about menstruation and the female organs.
Heavy and irregular bleeding can be a warning sign. It is best to make a gyn appt. I don't want to scare you. But, it is a sign of some major problems as well as some minor ones. I couldn't speculate. It is really a good idea to see the dr soon.