Third Wheel

I'm kinda having a friend problem. And I need your help!!! I am really close to one of my best friends. But lately I've been vacationing so I haven't seen her in a while, so whenever I Text her or call her to make plans it's "Oh I'm spinning the night with someone already.... Sorry maybe next time..." and this other friend is one of my friends too. I feel like a third wheel and I need advice on how to get back in this friendship. Any suggestions?! Please comment!