Sometime my I messages come thru from my email

I currently have a 6plus and previously had a 5s. Every now and then my iMessages come across in green, rather than blue and my friends advise that it is coming from my email address rather than my phone number. This happens even though we all use the same carrier and all have iPhones. I have gone to settings to see if I needed to change anything, but no luck. I usually have to shut down my phone, wait several minutes and restart it. No one else I know experiences this and it can't be a hardware issue because it's happened on 2 different devices for me. I also don't think it's my carrier, because again, no one else has this problem. it comes and it goes with no rhyme or reason. I can go weeks with no problem and then it just crops up. Thoughts? Suggestions? It is really quite annoying. HELP!