Michelle, welcome to TE.

Assuming you are eating enough and hydrating yourself sufficiently, if you have not been an athlete all your life, you might just be building muscle and not strong enough yet.
When I first started riding, even short rides over flat terrain totally wiped me out. With time (and i do mean years) I have gotten so that I can ride 40 easy miles in the morning and feel quite normal in the afternoon.. So, keep talking to us and keep riding.