Yeah, you are exploring too far too fast. You are going into areas where you are at the minimum level or below. I would say stop exploring and doing quests for now and simply farm for armor, weapons, & junk you can sell for caps and then buy ammo. I recall ammo being scarce up to around level 14, but I never actually ran out of ammo for my guns. I was primarily using pipe rifles because raiders use them, so they are more or less easy to get.... I do remember having a very hard time finding 10mm ammo though.
You want to get the Scrounger perk as soon as you can so that you can find more ammo; I think there are two levels for it. Cap Collector increases the amount of caps you can find; I think there are also two levels. Later on in the game if you decide you want to build settlements Cap Collector is a requirement to build shops.
I farmed the crash site of Skylane Flight 1981 which is south of Outpost Zimonja. Everytime I fast travel there enemies always respawns. I always crouch before going there so that I have a chance to sneak back and not engage in combat because there are always two enemy factions fight each other there. I would wait for the fighting to die down before going in to clean up whoever is left. Sometime one of the factions is the Minutemen so they are friendly. Be careful though because some of the enemies there can be pretty tough like the Gunners, or even worse, Child of Atom with their Gamma Guns which can cause a lot of radiation damage. But gamma guns are worth a lot of caps.
There is an ammo box, a safe, a duffel bag and plenty of suit cases to unlock. Plenty of chances to pick up clothing pieces that can boost your charisma if you wear them which will get you better prices when you trade with others. There are also 3 randomly generated weapons: one next to the duffel bag, one next to a secret compartment (master locksmith) that is in the lower deck of the wrecked plane (luggage compartment) and one in a bathroom in the upper deck.
While enemies respawn every time to fast travel there, all the containers and 3 weapons respawns after 480 in-game hours; or 2 weeks. The secret compartment does not respawn.