Hi there. I'm thinking what you felt may be your cervix and it is supposed to be there. :>) Or it possibly could be your hymen if you are a virgin. Our bodies change and our period changes as our hormones fluctuate. Having a change in flow and period and how much you cramp, etc. does definitely happen to a lot of girls. I'm not sure why you can't get a tampon in though. Do you think you are tensing at the time from either the cramps or anxiety about it? I've read that too if your menstrual flow is very light, it is harder to get a tampon in because there is not enough lubrication to make it glide in. Ibuprofen helps with cramps and swelling. Try taking some and see if this helps. And a doctor can do what is called a pelvic exam. It's time for you to have one of these. They are something that most women (as you get older) do every single year. Yep, it's that exam where you are in the stirrups with feet in air on bed. But it will reassure you that everything is just fine. Your gp may be able to do it or an ob/gyn is the specialty that does it the most. Can you talk to your mom about this?
Another possibility is that there is a forgotten tampon from your last period. Do make a doctor appointment (and don't be embarrassed if that is what it is -- 'lost tampon' appointments are not uncommon.) The reason I wonder this is because you have been able to use tampons fine for a year and now you can't get one in. It's a possibility, anyway.