Tailbone pain in pregnancy is usually due to ligament laxity, as a result of an increase in hormones. Women who didn't experience this in their first pregnancy can have this symptom in their subsequent pregnancies because their ligaments have already been stretched/loose once before.
The pain is often the worst when changing positions, such as standing up after sitting for a period of time. You can help reduce the discomfort by:
A - sitting/lying on an ice pack for 20 min at a time every few hours or so, for a couple of days.
B - avoid sitting on ALL hard surfaces (your pelvis will essentially mold to the surface that you sit on, making your "loose" tailbone hurt)
C - when you do sit, stick you bum out before you sit down, so you land on the backs of your legs rather than rocking back on you pelvis
D - when you get in and out of the car, back up to your seat, sit down, then swing both legs in together
If that stuff doesn't work, then you may benefit from seeing a chiropractor that works specifically with pregnant women. 30 weeks is a long time to have tailbone pain. You can do something about this.