MM, I think I had it in my first pregnancy too, but it was never really picked up - DS was 8lb 10oz, which is a relatively big baby for someone my build.
I had a GTT because I have polycystic ovarian syndrome which makes me more likely to get diabetes anyway, and one midwife said my sugars were a bit high, then a fortnight later another midwife said they weren't. However, the hospital have now lost my notes from that pregnancy, so I can't check.
This time round (I'm 39+3 today) I asked about having a GTT again at 28 weeks because I'd had one last time and also because I had loads of diabetes symptoms - I was waking in the night to drink a couple of pints of water, was totally shattered, breathless and looked pasty and ill. I felt MUCH worse than when I was pg with DS, really, really ill.
Started eating more sensibly (I'd been eating relatively well, but because I'd been feeling so tired, I had been giving myself sugar boosts mid-afternoon - you know, a soft drink here, a chocolate bar there, etc, etc) and measuring my blood sugars (isn't hard and doesn't hurt, just takes a bit of getting used to - well, it does if you're me and can't remember things from one minute to the next - kept having to write notes to remind myself to test - lol!) and felt heaps better within a few days.
It does sound strange that you didn't have the GTT and they're saying you did have GD though - can't say whether you did have it or not, obv, but definitely question them again about it. How many weeks pregnant are you? The GTT's done about 28 weeks. If you did have it last time, I think you are more likely to have it again, but only a small percentage of women need to take insulin to manage it - the vast majority are diet-controlled.