Something (bold) might be overriding the default style.
Open up your document, and choose the Selection tool. Now, click on the pasteboard, or nothing. You want no frame, no text or anything selected.
Look in your Character Styles Panel. (Window>Styles>Character Styles)
Make sure that [None] is active. If you have Bold or some other Character style chosen, when you have nothing selected on your page, then everything is getting that bold style as a default without you knowing it.
Then, do the same check for the Paragraph Styles panel.
Make sure it's using Basic Paragraph (or a specific style of your choice,) but make sure there isn't a "+" sitting next to your [basic Paragraph] If there is, then Bold (or something) was overriding your style and it's sometimes hard to get rid of it. Hold Option or Alt, then click on the word [Basic Paragraph} in the Paragraph Styles panel. This will clear out any overrides.
Now, make a new text frame and type in a caption, and apply your style.
hopefully that's it :-)