Not really sure...

It only happens when I'm sitting and not really doing anything at all. I suddenly feel my heart pumping oddly. I don't know how to explain it... It's like instead of beating, it's pounding for like 4-5 seconds and then it's normal again. It's happened maybe 3 times in the past 6 weeks and it really is when I'm just sitting there doing nothing  

It's never happened before so it MAY be pregnancy related? I have an appointment on the 19th but what do you think: should I see someone sooner?

It's normal. It could be anxiety or lack of cardio exercise (that's my problem lol)
Pregnancy doesn't usually mess with your heart until later on.

Might just be anxiety hun,but you do need to bring this up to your doctor when you go on the 19th. I wouldn't worry about it right now unless you develop other symptoms and it's really bothering you.

I don't feel anxious about anything though. That's the weird thing.... I know when I'm like that because I'm a total mess. With this, I'm just chilling and then I feel weird... Maybe I'm not used to the extra blood or something?  

It could just be heart arrhythmias or palpitations, I get them too but have done for a few years. Sometimes my heart will speed up for no reason and sometimes it will slow down without reason for cause. My Grandfather on mums side has the same thing, but since I've been checked twice, once in '09 and once with my current pregnancy (as advised by mu midwife just to be on the safe side) I know mine is nothing to worry over and that the rhythm of my heart just acts up from time to time (usually months to a year apart).

I would go and get it checked out either way, sometimes I can be nothing but that's not to say it may not be something more either.
All in all I am sure the doctors will tell you that it is nothing to worry over.