What is the cause behind mild tooth-ache?

Many things: A carious lesion, an abscess, a periodontal problem, bruxism, a loose filling, a fractured tooth. As you can see, can't definitively diagnose your problem over the internet. Call your local dentist for an evaluation.

Lots: Probably infected tooth (cavity), but can also be fractured tooth, malpositioned teeth, sinus infection, clenching or grinding, etc. Although toothache may abate, unless it is treated problem can come roaring back. Please call your dentist for an appointment before you wind up with one of those middle-of-the-night holiday weekend emergency mind-blowing toothaches.

It could be anything: This can be related to an area of decay, a faulty filling, a sensitive root surface, a loose or decayed crown, clenching or grinding, among other things. A dentist can determine which is most likely in your situation.