How bad is laying on your back??

So I'm a FTM and I've been told not to lay or sleep on my back but I am 34 weeks and I constantly wake up in the middle of the night flat on my back. It seems to be the only position I am actually comfortable in 😓 when I lay I my sides my back hurts so so bad and so does my stomach but when I lay on my back all the pain goes away and I feel so much better. How terrible is it to lay in your back? I really try not to but it seems to be the only way I can get relief! Thank you!