My bike shop sold me 700*20/28c Specialized inner Tubes for my 700*32 Trek SL5 Domane, and I got another flat first time out.

I checked tires, and wheels, nothing broken or sticking out.

Question:just coincidence? Should I be going for a larger inner tube? In general do I want my inner tube to be rated on the lower side (like this) or the upper side compare to my tire (like a 700*32-50)

Also any recs on tubes recommended?