About 2 months ago I biked out of my driveway, went about a quarter of a mile and I got a pinch-flat. I thought whatever, put a new one in and just went home to play basketball. Then a few weekends ago (after taking a break from riding) I went out to Dallas to bike around with buddies. About 2 miles in I get a pinch flat, replaced it with my last tube, then got another pinch flat in about another 2 miles. I looked at the tubes closely and oddly enough there was a break where the valve meets the rest of the tube perpindicularly. It was as if I could almost rip off the valve...at the very base. Is this common or have you ever heard of this happening before? Both those tubes were from reputable companies and I hate wasting all that money. Opinions?