Have I been undercutting myself?

As a web developer who has recently completed her first Flash project, I am in great need of some advice from all of you who have been in the field a lot longer than me.

I have already been paid for the site, but am concerned that I have under cut myself in pay. I worked approximately 18 hours on the project which entailed the site design, construction, 1 animated slide show, and an extensive intro splash page in Flash.

Can any of you offer suggestions as to what you, considering what I’ve described, might charge? Or, can you offer any references from where I might get and better idea myself?

I appreciate any help you can offer.