well- while everything is possible- somethings r more possible than others & n this case the thing that has the highest possibility is "physical strain" maybe u lifted something heavy weight or even normal weight but u moved suddenly or for a long period- might b driving for a long distance specially f combined with lack of enough sleep, might b heavy desk work working with papers or screens for a prolonged time specially f not using a comfortable chair, might b using smartphone for reading watching playing or socializing while ur head tilted toward the screen for a long period, might b watching tv with ur head tilted to one side or slept while setting watching tv & ur head got tilted, might b ur pillow or bed needs to b changed, also might b an injury from sports like football or so or from falling on the ground from relatively higher place, also not drinking enough water is bad for the body including muscles, probably it's a combination of the above things, hot water bath & a good massage for a couple of days usually solve the issue, f a pain continue for an entire week definitely doctor needs to b consulted, everyone should do the every 6 months regular checkup no matter how young & n case of elders every 3 months. great that u r better now & have a good day