Why Does It Hurt When We Aren't Together?

I work retail and it's that awful time of year when my store is open until midnight and we're given long shifts to accommodate the messes people make. I work full 8 hour shifts for the next three days, work an early shift Wed then have Thurs off, then back to two 8 hour days before. I spent the night with my boyfriend for four days straight (a length I haven't achieved before) and now that I won't really be able to see him until Wednesday, at least, I'm all emotional and my heart hurts. We've been together for nearly two years but I don't understand. My period just ended today but I'm still confused. I don't want to seem needy or clingy (though he says he doesn't care).