As others have said, palpitations in pregnancy are quite normal and nothing really to worry about. A lot of people have them pregnant or not, just that for most they don't even notice they are having them.
How to explain what your heart is doing. Basically, your heart beats, but somewhere in there, the valve going out forgets to open for some reason, so while your heart keeps on beating as normal, as the valve remains closed, even for second, this then causes a delay. As the valve opens and the blood is forced through, as more is pushing up behind it, it can cause the lightheaded feeling and the 'whoosing' you might feel and your heart rate may increase for a little while to accomadate what has happened.
I've suffered them for years and often get very large ones, where I feel everything stopping for 5 seconds plus, then get the rush of blood going through and the very lightheaded feeling that follows. I also get lots of the small blip ones. I used to find the ones that occured when lying in bed at night quite frightening as it makes me feel quite ill, but it always rights itself.
I am not sure how big yours are, if they just feel like a small blip, or you can actually feel your heartbeat stop then start and beat faster for 20 seconds or so till it rights itself. Either way, the key thing to do if you find them distressing is to just remember to keep breathing normally.
During my pregnancy I have had to see the cardiologist a few times because of my heart problems, have had an echo, ultrasound and a few other things (I have a murmur also) and he assured me that up to 3 palpitations a minute was acceptable. Also as I already know things like stress, constipation, senditary lifestyle (I am guessing you are not moving around so much atm) can all go toward them happening.
If you want to drop me a PM about it feel free