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That’s definitely odd, but don’t jump to conclusions just yet. Are you using a messaging app like Whatsapp or LINE? Sometimes things just go wrong with them, but not very often. If you are using the native SMS service, with most carriers you can go online and check your message history. If you can, do that and see if there is anything that looks out of place. Some rogue apps will send out texts, and this actually is something to be concerned about because they can cost you money. Install a good malware scanner like Lookout or AVG - you can get either for free from Google Play - and run a check on your system. Also, if there are any apps you installed just before this behavior started, uninstall them or at least review their permissions to see if they could be at fault. Hope it’s just a glitch, but better to check it out than ignore it.