My mouse is freaking out!

What's going on? All too often, when I use the left button of my mouse, it's minimizing pages. And, for example, if I were to click the "B" (bold) to bold-up a word using the vBulletin editor - I'd get text.

Why is my mouse freaking out like this all of the sudden? Is it something to do with my Mac? Or do I need a new mouse? I suppose I'm basically asking if this is common...

Again, this doesn't happen every time - but enough to really start bothering me...

Thanks in advance..

 

USB mouse.. A Dell.. Connected to my Apple keyboard. It's worked flawlessly since November.....