There’s a few possible underlying causes that could be checked.
The first thing I think of is gas buildup or constipation. It can be very uncomfortable and even painful for infants, and may show up only every few days or less often. If the baby’s stomach feels unusually firm to light pressure, it’s probably a digestive tract discomfort. Give the baby tummy time, move their legs in a bicycling motion, even try gently touching their knees to the elbow on the opposite side of their body (left knee to right elbow, and vice versa). If the pain/discomfort doesn’t subside after some gas release, or if gas doesn’t seem to be the problem, contact your pediatrician to be sure.
Not many babies get teeth this early, but it’s possible. Feel the gums to see if there are any tooth buds showing up. When my daughter was six months old, her teeth started showing up and she was miserable off and on, so I helped her with a breastmilk popsicle (formula would work too, of course!).
I just poured some pumped breastmilk into an ice cube tray and then held the frozen cube in a washcloth for her to nibble and suck on. It helped soothe the pain and reduce the swelling of her gums. Plus, it’s appropriate for consumption of a wide age range since it’s one ingredient – breastmilk or formula. Obviously, give the baby a break from it every couple minutes so their mouth doesn’t get damaged from the cold.
If the pediatrician doesn’t have an explanation for excessive fussiness, and you feel you’ve done everything you can, you might have just entered a phase. Yep. A real, borderline-worthless term that means so much yet remains so mysterious. It saves sanity on many days of exhausted parenting. There are sleep regressions, changes in feeding desires, growth spurts, bones are growing and skull bones are even moving and fusing, and your baby might just be generally uncomfortable sometimes. They might be missing mom or dad and need some extra cuddles. They may be reacting to an environmental change.
A nice, warm bath can work wonders. Let the baby listen to the running water while you get everything ready. In a rush, give them a good snuggle while you run the water from any faucet. It’s a soothing, whooshing, womb-imitating sound that calms many babies almost instantly.
An age-appropriate sleep sack can soothe baby when a caregiver can’t. Wrap em up, lay them down in a safe place and see if it helps. It imitates being embraced, but some babies aren’t fooled by it and want the REAL deal.
This, too, shall pass. Don’t hesitate to contact your pediatrician if you’re concerned. They’ve heard MUCH more ridiculous things.