@jeff98682 I'll admit as a teenager I had pinworms and never told my mother. I don't know why, probably because they didn't cause pain or any other symptoms that were more concerning than the embarrassment I felt. Eventually they did go away, even though I didn't get treated, but I had them for a long long time.
Try not to overthink her choice to keep it private despite your close, open relationship. She DID decide to tell you, you didn't learn about it thru a third party or accidentally - which means she was open and honest with you, just in her own time. Remember, you only had to hear the words - she had to SAY them. You mention how grossed out you felt, take a moment to step into her shoes and think about how she felt in the moment. I imagine she felt embarrassed, disgusting, anxious and a little nauseated too. Those feelings are probably why she needed time to reveal her 'secret' and are completely unrelated to your close relationship.
She was probably hoping they'd just go away and subconsciously ignoring them. Adults do that all the time with minor health ailments.