Heya.. Yes, your question does make sense. And no, it's no trouble at all. Why would it be? There's no straight-forward answer to it though.
It's not always the same. Sometimes it's just as you said, a clear idea forming in my mind, triggered by something I saw or heard maybe. This can be an artwork I see on the internet, that makes me want to try something similar. Or it can be something seemingly insignificant or unrelated, a line or shape that looks like a face or whatever else (like things one might see in a cloud) that would make a neat little drawing. Other times it doesn't take any "external input" and instead there's just the desire or a spontaneous idea to draw this or that => "hey, I've never drawn an afro-zombie-cyborg pizza delivery dude". Often times when I sit down with an idea in my head and start scribbling, I hit a deadend, can't really get the idea on paper in a satisfactory fashion. It's actually a very common thing for me to come up with something solid after a very frustrating first hour(s) of trial and error and quite often I end up with something completely different from what I had originally thought about. To be honest, I feel kind of uninspired, uncreative most of the time (though for some odd reason people won't believe me.. haha). Sometimes that's bothering me - but mostly I'm fine with it. I'm not trying to reinvent the wheel with every single picture I make. There are a few subjects that I stick to and that appear quite frequently throughout my entire gallery; mostly I draw characters, rarely full scenes or environments and stuff. Cute girls, weird skulls, people with big blades, gloves, collars and hoods, floating bands - that's what I like to draw most, what I rearrange and draw time and time again. I'm simply trying to do what I do and improve my style along the way. It's a slow process, no courses, no school or anybody formally teaching me, just natural development by doing things and reading up on the few points I can't figure out for myself. This year I finally began drawing on colored cardboard with ink and white pencil. This certainly doesn't sound very exciting but it's been something I've wanted to try for a long time. It was fun and with it came some ideas for new images that simply felt right to do in that style. So, inspiration came simply by trying something new... well, partially new anyway since I have used inks as well as pencils before, just not in combination. Anyway, I think you get the point :)
Now, can I give advice on how to get inspiration? No, unfortunately I can't. If somebody can, I'd love to hear it myself :D The only advice I can think of is something I've said several times in the past: just keep doing stuff. Experiment, try to find solutions for things you seem to be unable to do yet, look at other people's works and techniques (something I've neglected for a very long time). I think nowadays it's easier than ever with the internet, movies, comics, games and all that to find inspiration. Just don't try to force anything. If one day you can't get anything on paper, put the pencil away and try again the next day. Trying to force inspiration seems to have quite the opposite, negative effect.. at least for me it works that way. And don't feel too upset if things take more time than you expected. Ah well, don't know what else to say here. I'm afraid that sounded kinda esoteric but hopefully it answered your question anyway. Thanks for the compliments :)