Hi,
Really sorry to hear of your loss. It is really difficult to know for sure and even the experts are divided on the subject.
When I had my m/c last year I had a badly infected finger nailbed. The start of the infection must have coincided roughly with discovering I was pregnant (I had trapped the finger in a door a month previously). Had 2 courses of antibiotics (which didn't work) but then I m/c. Docs said it would have been nothing to do with the infection or the antibiotics but I remain unconvinced because the infection continued for a further 3 months before I had surgery; have never had trouble conceiving before but it just didn't happen whilst I had that infection! However, 2 weeks after the surgery (nailbed removed/ cautouised) I conceived!!
I've just had to accept that it is one of those things I'll never know (beat myself up for months about it and it has taken a long time to get over that). All I can say really is don't be too hard on yourself. It could well be nothing to do with the flu anyway (I've had a constant string of horrible viruses since being about 7 weeks this time and have been fine) but even if it was, it wasn't your fault, it really wasn't. You're probably sick of people saying it, but it does get easier in time, honest.
All the best and take care