I'm 70 and frequently have an itchy tongue. I've learned that sometimes it's because I've eaten something to which I'm allergic, sometimes it's because I have a canker sore along with a cold, and lately it's been because I've been sucking on my tongue. I'm not sure which came first. It may have been slightly itching which started the sucking or I may have started sucking and it became itchy with red dots. I do know that when I've successfully stopped sucking on it the itching goes away.
I've been to the doctor about it and they've all said they don't know what causes it and there is nothing that can be done. It could be such and such, all listed above.
I doubt yours has anything to do with being pregnant. Except that your body chemistry may be different and that can happen even when one is not pregnant, such as when one has a cold.