Hi Maria,
is that true for all letters "a" in the paragraph you did not type?
To test that: Copy/paste an "a" from the text editor unformatted to that paragraph.
If the pasted "a" changes to capital "a" it's very likely that there is a GREP style applied to the paragraph that targets all letters "a" and applies a character style that formats the found "a" to a capital letter.
Regards,
Uwe Laubender
( ACP )