1.) Sometimes your baby can be fussy because he's tired, hungry, gassy, overstimulated, needs a diaper change, or just wants to be held.
2.) Or the little things may bother her, like a hair wrapped around her finger, or the way the cat smells.
3.) If you're breastfeeding, then maybe it's something in your diet that's giving him a tummyache. If you're formula feeding, it could be the brand of milk he doesn't like.
4.) Maybe she's overtired. To help put her to sleep, it can help to take her on a car ride, or to turn a vacuum on, as this can soothe babies right away.
5.) It could be possible that your baby is colic or has reflux. This can cause him to cry.
6.) If something just doesn't feel right to you, then she could be getting sick. Check her temperature regularly.
Good luck!