Recipient receives MY texts from a different number

[thbc1391]

My girlfriend has been receiving my texts to her just fine but they appear to her from my number but with a different area code and therefore do not show up under my contact name, so instead of her seeing my name she will see ex. 515-555-5555. Also, she can not reply to that number at all, so basically she has to access two different text convos to talk to me. Any idea why my area code would be changed and why it would be sending from a separate number? Any ideas on how to fix?