Why does my eye contact keep moving when I blink?

I bought a pair of blue coloured eye contacts last month from a local store. The contact lenses are certified by the various medical associations (although I can't remember their names), and I even double-checked them online. They're safe, and a few of my friends have used similar ones from the same place without hassle.

For the past month, I've been wearing them every so often (I'm trying to make my eyes get used to them for Comic Con in September) without any real problems. They go in my eye, I blink a few times, pat my eyelids to make sure they're adjusted, and they fit perfectly fine. No problems (apart from an ever so slight cloudiness at either side of my vision, but I'm told that's normal). But since last week, I put them in, and the contact on my left eye felt Really uncomfortable. I cleaned it, made sure it wasn't damaged, and put it back in. It went in, still felt uncomfortable. I blinked, and noticed that it was kind of trying to slide off my iris. It kept jolting up diagonally by about half a centimeter, then would go back into place. Every time I blink it moves, which it never did before, and it's really uncomfortable. Does anyone know why this is? It's just suddenly started happening and I don't know why. I've stopped wearing them since the problem began.

I've seen people ask this sort of question before, but it's usually been under different circumstances to my current situation, so I thought I'd ask again. Thanks for reading.

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