What could be the cause of irregular periods?

Hello doctor,

I am a 19-year-old female, 5.3 feet tall, and weigh about 125 pounds. I do not have any medications. I do take multivitamins and several other kinds of vitamins and supplements. I also do not have any known allergies or medical conditions. My issues started last month when my period came. I noticed something was off because the blood was the darkest I had ever seen; it was almost black and there seemed to be more clotting than usual although not large clots. My period was also very light compared to how it is usually and it was accompanied by something that I had never had before. That period ended after five days and I put it down to it being a fluke and waited for the bloating to dissipate. The bloating went down some, but I was surprised a week later to see I had started my period again. This time the period was much more like my usual one. The blood was a bright red and there were hardly any clots. But this one seemed to be a bit lighter as well and the bloating did not go away. It has now been two weeks since the last period and the bloating has stayed. My stomach and hips carry a lot more weight than they did before; it was like overnight I just gained a bunch of weight that will not go away. Other than these symptoms I feel fine, but it has been bothering me since it is so unusual. Up until now, my period came around the same time each month with the only issue being occasional painful defecation. There are only three reasons I can think of as to why this has been happening:

1. I either have some underlying medical condition that has suddenly reared its head.

2. Or because my dad started having me take these "natural" vitamins and supplements (to treat my anxiety he says) a little bit before I started having these issues. Maybe these just are not working well with my body.

3. Or maybe it is because I could be pregnant which I highly doubt as I have never had any sexual encounter with anyone at any time in my life. So there is about zero chance that I am pregnant, but the anxiety that plagues me would not let me rule it out.

So my question to you is: What could be causing these symptoms? Do you think it could possibly be one of my reasons? A combination of them? Or do you think it could be something else entirely? Please help.

Thank you.