Just let me say, this is TOTALLY NORMAL!! It's usually called Seperation Anxiety and it usually begins with the age/stage of being more mobile and aware of the world around us. My son experienced this and the explaination I got was, as his awareness grows, so do his fears and those fears are coming from the fact that he hasn't learned that if something (like Mommy) is not in his line of sight, it has disappeared forever...AND WHO WILL TAKE CARE OF ME!?!?
There are things you can do now, to help your son learn (not train) that Mommy is ALWAYS there no matter what, without breaking your back. This is not manipulation. I don't know why people think a baby knows how to manipulate an adult.
What I did was make him a part of the day...if I was cooking, I'd sit him in his high chair, boncy seat or his bumbo on a blanket nearby and let him watch what I was doing. I got him some pots and pans from Ikea and let him cook too. I would explain what I was doing. Washing the veggies, cooking the eggs...this will also help with language development. Same thing for washing the dishes...gave him a rag and plastic container that was wet and had him help me and he loved it.
Laundry meant we'd sit on a blanket on the floor and I show him how to sort socks by color and so on...he's still a very good sock sorter at 2.5. As our kids get older, being Mommy's helper is such a HUGE deal!! My son now calls himself Mommy's 'super helper' and it makes him so proud.
Another thing I started doing when my son was 9 months old was playing peek a boo. While it seems really simple, it is great for easing Seperation Anxiety. I would sit him on the floor and use a blanket, on the counter while in his bouncy chair or in his high chair. Make it a game and explain that Mommy WILL always come back, no matter what. Start explaining and talking with you little one NOW and you will have layed the ground work for good communication as he becomes a toddler. At this age, I don't think people talk enough to their babies and it helps SO much. Use a calm soothing voice and explain that everything is fine and no matter where Mommy goes Mommy will be back. Start slowly with putting him in the high chair and leaving the room for a second and popping back in with a smile and a laugh saying 'Mommy's here!'. Then, increase the time and eventually he'll get the drift and know that nothing will keep Mommy from him.
It's funny because we say 'out of sight, out of mind' but, for babies with this kind of anxiety it's 'out of sight, out of existence and who is going to take care of ME'. Remember while he learns that there are things that exist out of his line of sight, he's going to need some guidance and patience but, you don't have to break your back with carrying him around ALL day.
Oh, one more thing!! My Mom (former teacher) used to place familiar things out of sight and place a type of hide and seek game for them. Like stuffed animals or toys and then, she'd say things like 'where'd the dinosaur go, I don't see him' and then, she'd take him looking for the dino and when they found him 'she's say there's the dinosaur, he's in the den'. I think this little game helped a lot in his older phase of Seperation Anxiety...yes, it can happen at various ages/stages and my son has repeated it almost with every BIG milestone change.
Just remember be calm, be patient and don't forget to lauch!