I don't see how. People have been using bass amps for guitar for as long as there have been electric guitars, and bass amps to plug them into. The Ampeg SVT was originally designed as a 300 Watt bass amp. The Rolling Stones weren't having any of that, of course. They used them for guitar as well as bass when they debuted the amp on their '71 tour. So, in general, no - playing guitar through a bass amp will not harm the amp.
Down to your individual case, there may be something going on that you didn't notice before, or that has been hastened along by the different settings you use while playing guitar instead of bass. Unlikely, but in a universe where anything is possible, you gotta consider that yours is the one situation that came out different.
From an electronics point of view, an audio signal is an audio signal, and an amp works with audio signals. The frequecy content may be different, and the levels may be diferent, but the amp either passes or doesn't pass the frequencies, and no off-the-shelf pedal should be putting out voltage levels capable of causing physical damage to the input. The sound would simply be more distorted if it was too strong for the amp to reproduce cleanly. So if there's distortion now that wasn't there before, your amp has a problem that would have been there regardless of whether you were using it with a bass, a guitar, an electric piano, or a microphone. OK?
Mechanically, the vibration from the speakers working at higher frequencies than 'normal' may have caused something to come loose. So if its a tube amp, make sure all of the tubes are seated properly in their sockets. Check all of your controls to see if any of the pots are 'scratchy'. Check your cables for loose connections. Check your cab for loose panels, connectors or grilles. Try relocating the cab. Point it in a slightly different direction. I remember when one of my bands set up for rehearsals in our bass player's basement, there was a small crack in the concrete at one corner of the window. Six months later, that crack ran all the way down the wall to the floor. So if you've had the amp and speakers in the same position while you've been playing it for however long, you may have shaken something loose in the room. Really.
