Follow your heart! If you want to work for a startup (for no matter reason, money is a completely valid point) - do it! If you want to stay and help your company grow - do it! It's life, things change.
Thank you CReber. I do like my current company and the people I work with. I think I'm going to let my boss know I have an offer and that I don't REALLY want to leave, and try to get a little bump in my pay anyway. My heart is telling me to stay.