What should I do when I start having back pain every 10min after playing? I sit down and change position but the pain continues. Every player has to support the weight of the guitar if playing electric guitar right? Are there lightweight guitars? What solutions are there that don't involve medicine?
Without seeing a picture of your playing position or watching your technique, it is difficult to offer proper advice. Let me preface my answer by saying that it would be a good idea to see someone about it - a friend of yours who teaches guitar, a guitar professor at a local university, even a doctor such as a physical therapist could help you and probably give you more personalized information.
That said, after reading your question, a few things came to mind:
Players typically do not realize that their guitars are closer to their chests when sitting as opposed to standing. All of the weight of the guitar is on the lap and the player is usually slouched to some degree over the instrument. Standing with a low-hanging guitar like what is seen with many Metal players causes the back to support the majority of the weight of the guitar and leads to improper player position.
This can be remedied by adjusting the guitar straps so that your guitar is in the same position standing as sitting. When you stand up, it should be fairly high up on your torso. It will look a little goofy, but the weight stays off the arms and that is good.
If you purchase a wider guitar strap, it will distribute the weight of the guitar more evenly across your shoulders, and therefore incite less back pain.
Straining the muscles in your arms can include the shoulder muscles in your back. Tenseness when playing is a no-no. You should be relaxed all of the time.
If you have a herniated disk, then the weight of the guitar could be triggering your pain.
If we know where in your back the pain is, that can help narrow down the cause of the pain.
Regarding the weight of guitars, most guitars are fairly lightweight - a notable exception being the Fender Squire Stratocaster and some Jaguar models (unfortunately!) These guitars can be in the 8-10lb range. Other guitars that are made from different materials, like Basswood are very light and are more in the 5-6lb range. Acoustic guitars are obviously quite light, but the weight of those guitars is found in the neck; which requires appropriate compensation.
Hope that helps.
If you're sitting down, the weight of the guitar should be on your lap, not your back. So I'm putting this one to posture. Like your mom said, sit up straight.
To answer some of the other questions, yeah, there exist lighter guitars. Les Pauls are solid chunks of mahogany with a maple cap, and the old school ones are notoriously heavy, while some Telecasters are swamp ash, which tends to be pretty light. Some instruments are chambered to decrease their weight (certain modern Les Pauls, for example.) You can find light instruments, but by your description, that's not the issue.
In addition to all the good answers already provided by our community, I would give this advice that can be applied to any activity that tends to involve sitting for long periods: