My dd was always head down on all u/s check ups. You need not worry about that.
Congrats on a healthy baby!
My baby was head down too at my last ultrasound (20 weeks). Not really sure how she's been laying before, as I never really thought about asking until last time. Me and my DH were actually thinking that was a good thing, as our neighbor had to have a c-section as her baby would never turn. But I also know that this is usually the time in our pregnancies when the baby is the most active, since there's still enough room to move around quite a bit. So I'm sure their positions change quite often at this point.
Mine's a thumb sucker, too, by the way. The last two times I had ultrasounds, she was sucking her thumb non-stop, only taking a quick break to move around once the ultrasound probe got a little bit too close for comfort ;-)
I was checking your profile, but didn't see any photos... Hopefully the baby will be a little more cooperative next time, so you can find out if it's a boy or a girl!
Best of luck with everything!
:-)