Attending a work party, but not an employee?

My Junior High-school that I worked at as an ALT until last month is having a sort of end-of-school year party for all of the teachers leaving the school. A note with all of the party information was put on my desk a few weeks before my last day, and I was asked by two different teachers on two separate occasions if I was coming. I said that I would, but I really wanted to ask if it was okay to come, since I would not be working for the school anymore at the time of the party. I didn't get a chance to ask, and now I am unsure if I should go. Would my old co-workers find it weird if I showed up? Or am I just letting my social anxiety get in the way?

You were formally (in writing) invited and you were also verbally invited. I don't see any reason to think you'd not be welcome.

You were working there until recently, I don't see any reason why your presence would be unusual.

Working until February is almost the same thing as working until March. I'm sure they asked just so that you'd know you're one of the team.

It's a normal invitation at Japanese work environment.
But, if you need to pay to attend the party, you can decline the offer of attendance because you are no longer working there.

Thank-you for all your help guys! I guess it's just my social anxiety getting in the way. I think once I get there and get settled, I should be fine.

You were invited there is no reason you shouldn't go.