Hope my story is encouraging and helps in some way. I am now 18 wks pg with #2 having ttc for nearly 3 years. I also had a diagnosis of secondary unexplained infertility. DD (now 5)was conceived immediately and like lots of you I had just assumed we could pick our moment for #2 .
All tests were normal and I was ovulating normally too. Had one go at IUI which was abandoned as I was massively overstimulated - but at least it did show I had a cyst on my right ovary which no one had spotted at previous scans.
We were contemplating IVF (very half-heartedly) so decided we would invest in some private investigations first - still loads less expensive. I went on to have a laparoscopy where they found I had endometriosis and a haemorrhagic ovarian cyst with about 2 years worth of 'old blood' in it (sorry if tmi!). This can apparently be catastrophic to fertility in several ways not least because it causes inflammatory substances to kill off sperm. This was lasered away at the same time.
Anyway, after a month on the Pill (to prevent my ovaries from going mad before they were healed) and then 3/4 more months ttc and I got pg.
The only other thing we did differently that month was have loads of sex - every day for about 9 days which incidentally we had tried before to no avail - plus, and this my be irrelevant, I had lost lots of weight over Xmas because I had 'flu. (Wasn't technically overweight to begin with but at the upper end for me. Some women's hormones are more affected by abdominal fat and I lost a fair bit of that...)
Anyway, enough rambling. My point is secondary infertility is not unexplained until you have been properly investigated (my very nice consultant pointed this out). I felt rushed into IVF - age was a big factor, I am now 40 - and somewhere deep down 'knew' there must be a reason.
The years of ttc were horrible and I genuinely sympathise with everyone here. I felt resentful, unable to congratulate pg friends or chat normally about their babies, reluctant to go to places where babies would be etc. I can now do all these things normally (so none of us has become inherently mean or unpleasant!) but I have been sharing the news, I hope, in tactful ways in the knowledge that it is not likely to be 100% welcomed by everyone. Some people were incredibly crass when I was on the receiving end IMO.
Anyway, please get properly investigated and don't give up. I was truly sick of 'sex for procreation', DH and I bickered constantly, and it all seemed utterly pointless after nearly 3 years. But it wasn't pointless and I am very grateful.