A.C.
My son (who is 6 now) went through the same thing about the same age. He had always been quite sensitive and preferred "Mommy" to others, but his fears and attachment to me really increased/became more noticeable around that time and continued until he was almost 5 (then re-emerged briefly at the start of kindergarten, then got better again). I've always worked and he had always been in daycare, so it wasn't like he wasn't used to being separated during the day. He became afraid of everything - noises, strange people, kiddie videos (even "Land Before Time" which is about the tamest thing out there), and any books that had the slightest negative chracter or story-line. And he started having nightmares and being afraid to go to bed. I tried to equip him with nightlights, a flashlight, stuffed animals, a "dream catcher" to hang on his window. Sometimes those things helped and sometimes they didn't. I often would just "cuddle" next to him in his bed until he fell asleep.
At some point, he just seemed to grow out of it. So, your daughter's sudden sensitivity doesn't seem strange to me at all(though it's always worth a call to the Dr. to rule things out). I realize in hindsight my son was just going through a phase. He has always been sensitive and very aware of everything going on around him and I think that age (3-5 or so) is just when their little psyche's have the hardest time making sense of all the stimuli around them. Just give her the extra loves and reassurance she needs and assure her that she won't always find things so scary. My son seems to handle school and the pressures of life just fine now for his age.
Good luck!