mmm this will be tricky to explain since the old tube is now in somewhere in the city trash. There was a hole right along the middle, about 3 inches long and about as wide as the tube itself. It was roughly opposite to where the valve is located. Although I was told that something could've been potentially wrong with the rim or the tube had been poorly seated, the mechanic found nothing wrong with the rim and he didn't mention anything wrong with the bike. My bike has received its first tune-up, and as of now, I am riding with both tires at 70 psi (The way it left the shop). The bike feels very good and the tires do not squish under pressure.

I probably should mention again that when I inflated to 100 psi, the pump piston started to feel incredibly hard - it was quite an effort to do that. (Is that normal, anyway?)