Should I go to a hen party if I wasn't her first choice?

I have been asked to go on a hen do by a colleague at work, however, I wasn't her first choice. This colleague I have history with, I went to primary and secondary school with her. I would say we get on well but we don't meet socially outside of work very often, text etc. I found out a few weeks ago she was going to go away for a hen weekend, I wasn't asked but it did not bother me at all. Since then one girl has dropped out and she has asked me to go. She explained that there were only 10 spaces in the villa and she could only invite a max of 10 people. At first I wasn't bothered that I wasn't in her top 10 and was up for going, like I said we are not that close. However, since being asked I have wondered how I will come across. Will I look like a mug stepping in? Have I only been asked to make up numbers? What should I do go or not? I am now worried that if I say no I will look like I am being being funny and was offended that I wasn't invited in the first place. I just wished I hadn't been invited at all, even as a replacement! Please help me decide what I should do.

Thank you @wintersun, what you have said makes sense to me. My first instinct was to go. I thought why not, have a bit of fun. There are two other girls going that I know, so it would be fun. However, when I told a family member they told me they wouldn't go if it was them, so it made me think about the situation and how I would come across. I think I might regret it if i don't go, but don't want to feel awkward.