When to go back to the eye doctor.?

I have recently noticed I can't see as clearly as I feel like I should be. I already wear contacts and I got a new prescription in May. I had been thinking I couldn't see as clearly as I had been so I did two of those online eye tests and one told me I was at 20/50 and the other I couldn't read the 20/20 line. Should I go back and have my eyes checked again? It seems odd that they would change this suddenly especially since I am 21 and my eyes should be stabilizing. But I have other pairs of contacts that I haven't used yet and I don't want them to go to waste. What would you do?