My Pregnant Cats Belly

Question
Dear Doctor Ray.
I've a question for you regarding my pregnant cat's belly . well , Princess is a 2 & half years old white Persian queen.she's pregnant for 3th time nowadays .she's about 40 days pregnant actually.
but something about her belly has always made be confused . you see.....naturally her belly has enlarged . but , in different different parts of the day , her belly's size is completely different . for example , when she wakes up after a long sleep , her belly looks as big as i thinks that its going to blast ! but when she's awake for a long time  playing and running , her belly turns much smaller than before.(it is actually a big belly still I must say , but in comparison to the same belly when she is asleep or just waken-up , her belly sounds much smaller to me.)
well , I just want to know what is the scientific cause of this ?!
i must add something else also . at her 2 previous pregnancies also she had the same condition . and both pregnancies led to healthy kittens . i know that there is not any problem inside , but i just want to know why is it like this?
by the way sorry for poor English of mine.
Thank you a lot .
Soheil From Iran.

Answer
Hi Soheil
There is no problem but this is a delicate thing to describe.
Remember when you get up in the morning and need the toilet to pass more than wee, well this has built up over night whilst you are sleeping. Your food has been digested and all the waste is now ready to leave your body.
Your cat is in the same situation. She has been feeding during the night and the food has built up in her stomach and intestines. In a normal situation you would not see the product inside because it would be hidden in her abdomen area, but since she is pregnant, the contents are now seen more clearly as they have nowhere to hide, because the kittens have taken up the room inside her.
Once she goes to the toilet and passes the waste her stomach resumes its normal size and seems to disappear.
I hope this helps

Kind Regards

Doctor Ray