Help.
So who is this Arminius?
And why am I not just called a CHRSTIAN?
Ok, I will give you a VERY condensed history lesson.
Basically the majority visible external Christian church gradually morphed into Roman Catholics and Eastern Orthodox over the years by the time Calvin showed up. This is not to say that just because they called themselves Christian that made them Christian, nor is it to say that there were literally no other Christians, but the RCC and EO became the majority by the mid 1500s. There were also other Christian groups like the Anabaptists who didn't like the RCC at all, and were named for their rejection of infant baptism and belief that everyone should make a choice, and were heavily persecuted and often killed and tortured by the RCC.
Now Luther and some others felt like the RCC church needed reforming, because in fact, the RCC was full of unbiblical teaching. However, it was intimidating going against a really big and powerful church that everyone looked up to, so Luther and some others started gradually "protesting" different doctrines, and gradually more and more people broke away and stated where they differed from the RCC, eventually being called "Protestants." Most of them were Calvinist for some reason that is not readily explainable. Now weirdly, these Calvinists ended up not liking the Anabaptists either, even though you think they would have some sympathy and understand them.
Okay, I know this a long setup, but you really can't get the big picture without it, lol. So, after Calvin wrote a bunch of stuff and codified Calvinism, then later some of the group didn't like his emphasis on removing free will and blaming sin on God. So Arminius was the guy who came along and challenged Calvinism (along with others, but he was kind of a leader and got the most famous). Then Arminius made his own five points in opposition to the five points of TULIP. Basically, ever since then it's kind of become shorthand for "Free will versus Divine determinism" to say "Arminianism versus Calvinism," and people argued it heatedly for hundreds of years.
There is nothing wrong with "just being called a Christian." Many Calvinists also want to just call themselves Christian and don't particularly like to be called Calvinists. However, labels serve a function. We use the "Trinity" even though that word isn't in the Bible so people quickly understand what we are saying, that we believe God is three Persons in one Being. So doctrinal labels can just be a quick way to let people know what kind of theology or belief we have, whether we believe that real free will exists, or whether we believe that God's divine decree determines all the choices of all people. And instead of writing that all out every single time, you can just say "I have Calvinist/Arminian" theology.
You can take all these data points and pretty much explore them ad infinitum, it's a pretty deep rabbit hole, but that is the extremely abbreviated version for you. There are many people still today who just hate labels for some reason, and just don't like the feel of them, so they refuse to call themselves any particular theology. But I don't think there's really any good reason not to use "shortcuts" to just say what style of belief you have. I very closely align with what is called "Classical Arminian Theology."