emeraldgirl not a silly question at all! Buy maternity stuff as soon as you feel you need it or start feeling uncomfortable. You sound like me - I was feeling very uncomfortable in most of my regular clothes by week 12. In fact I couldn't wear any of my normal jeans by week 8!! I was/felt quite bloated in the first trimester, and the waistband/button of jeans around that area made my nausea worse. I felt bad that I was needing maternity gear already, but in hindsight that was just silly. Your body is going through a huge amount of changes right now. Every woman is different. I know some people don't need maternity gear till like 25 weeks or something, but I was NOT one of those women - my body wanted to bloat and expand from the moment I got that BFP!
One tip though, don't buy very expensive stuff right now - New Look, H&M stuff would be fine. I'm at 33 weeks now and I find that lots of my stuff I bought around the 8-15 week mark doesn't fit me anymore.
I had a bit of difficulty finding maternity trousers that I really liked (but I have that problem with trousers anyway non-pregnant) - I found an alright cheapish pair for work from New Look, and a way nicer pair on sale at BoobDesign.com. As for jeans, got some FAB skinny jeans from New Look (now waaaay to small for my bloated legs) and some good skinny ones from Mamas & Papas too. H&M do excellent black stretchy lyrca pant things - they are not really trackpants or leggings, nor are they "proper" black pants but they're pretty good for most purposes really! Sounds like maybe something that would be good for you? I actually bought a pair of these at 7 weeks because I was flying longhaul and needed something comfy - they have been BRILLIANT! They are over-the-bump but that part can be folded down to hip level IYSWIM, so really versatile in the early stages. They were about 15 quid I think.
Unless your boobs really swell up you should be fine with tops for a while. I got good basic tops (tees and camisoles) from Next, New Look and BoobDesign. On the boob front, don't be scared to go and get yourself measured for a larger bra size ASAP if you're finding your current bras uncomfortable. Such a relief to be in a comfy bra.
Get thee shopping!! You will feel so much better and more comfy. :)