I'm 28 and I've never had a proper girlfriend. I was a bit of a nerd in school and didn't started late.

Anyway - I've been with quite a few women and dated quite a few short term but never really had a serious relationship.

The last two women I've been with have quizzed me about this...
Examples...
How come you don't have a girlfriend?
How long are you single?
What been your longest relationship?

Any of you find yourselves in similar situations? Should I just make stuff up?

when i was in your position, i typically saved that info till it didnt matter, so if they are asking you that and your not already in a relationship or dating, just parry with something like, I dont like to talk about that part of my past, or "why do you want to know" in a flirty kinda way. or you can be post-player about it and go with "when i was younger i thought i was the shit and just kinda went from girl to girl and stayed away from commitment, but i have grown up now." I typically dont condone lying, but i think in your case its ok. just dont let a small lie like that lead to bigger, badder lies, you know what i mean? just answer with w/e and move on.

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The first question - How come you don't have a girlfriend?

I've been asked this question a couple of times, always find it awkward to answer. (The other two questions I answered about a year.)

One time was lying in bed with a girl, the other was with a girl at work in a canteen. On both occasions I got nervous /embarrassed. Probably should have just said I'm happy with short term / easy going relationships right now.

"How come you don't have a girlfriend?"
Possible answers:
"Who said I didn't have one?" in a joking manner (Probably best to ensure that she knows that you're single and that you're joking, no need to scare her away )
"Just taking things slowly right now"
"Haven't met the right person yet"
"I've just come out of something long-term and I'm taking things slow"
"My mysterious past usually scares girls away" (do your evil-I'll-take-over-the-world-laugh just after this )

Loads of options really. If you have a proper excuse (just got out of something, e.g.) just be honest about it, otherwise try just bouncing back another question, or making a joke about it.
Either way, stay confident. If a girl asks you stuff like that she's obviously interested (she wouldn't be quizzing you on gf stuff otherwise), you don't want to blow it by suddenly getting all uncomfortable about it.