You would definitely know if you were penetrated.you don't need to be fully penetrated past the sphincter to call it an exposure , If his penis had gone into your anus, you've had a risk and need to test. I'm sure you're pretty well aware of the testing period and recommendations. If there was no penetration, your risk doesn't exist.
Hiv isn't diagnosed by symptoms, the only way is to test. If you truly cannot remember, I suggest you to test and stop having unprotected sex with anyone until your test results are out. Better to know than to live in fear and uncertainty. Be well
"5-6 days later she came down with a really bad flu and sore throat which she .. " - First of all, ARS symptoms do not usually appear this soon. So, in all likelihood, what your girlfriend experienced was just a common flu.
I can understand that you are now unable to tell for sure if there was a penetration or not. Dude, you would have known if he had actually penetrated you. Getting an erect penis inside is very different from just getting a finger inside. You would certainly have known the difference. This is just your mind playing a trick on you.
Now think of it this way. In order for you to get HIV, the other guy should have been HIV +ve, the probability for which is very low. Secondly, he should have penetrated you. Which, am sure never happened. Even if he did, the probability that you will get it from a single exposure is fairly low. So, the odds are largely in your favour.
Don't worry. Anxiety can cause a lot of symptoms and this might lead to further anxiety. Just relax. For peace of mind, you can get tested at the 90 day mark.