My eyes turning red all of a sudden?

I've been wearing contacts, circle lenses to be exact, for 6 years, since I was 13 all the way to now; 19. I never had a problem with irritable, red, or swollen eyes/scleras.
Up til this month, a week ago I wore circle lenses to work and kept them in for only 3 hours yet when I was about to take them off I notice ONLY my RIGHT eye was red, my left eye was completely normal/how it usually is. The thing is, I don't notice it turning red like throughout the time I was wearing my contacts my eyes didn't feel itchy, swollen, irritated, hurt, strained, or tired. It felt how it usually does; normal!
I thought it was like a freak/one time thing so I laid off contacts for the rest of that week and today I wore contacts again since a week ago, BUT my left eye is still turning red and now my right eye is kind of red. Not even eye drops help either /:
So what's the big deal?
This never used to happen even when I used to wear my circle lenses/contacts for 8+ hours straight, and I did this majority of the time, it never did turn red up til this month?

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