While it would be possible for two sentences to fill an 8.5x11 text box, it would have to be either two very long sentences, or a very large font, so I think its more likely that the OP hit the enter button instead of the return key (which on a Windows machine is also called "enter" but is the wide key to the right of the home row).
KevC:After you have pressed "enter" where is your cursor? I'm guessing its right at the bottom of the text box, or even disappeared out of it
The enter key will advance the cursor to the next column. In a one-column text box, that means the next box in the chain. If there is no next box in the chain, you will get the overset text indicator. The cursor wouldn't be at the bottom of the box, it would be within a part of the story that is not currently in any box.