The more muscle you add, the easier it is to stay lean, because all muscle burns calories at REST. Resistance training is the ONLY training that will raise your metabolic rate as you grow older. Resistance training also strengthens your bones in a remarkable way.
You can lose 10 pounds of WATER over just a day, or two. You don't lose two pounds of fat in a week, etc....that's just glycogen deloading.
The most important thing is to eat; to fuel the furnace.
At your very low body weight (5'9", and 130#) the LAST THING you need to be doing is depriving yourself of calories. You're extremely diminutive, and unless you like being small (I note you said that already), the whole notion of reduction makes absolutely no sense. EAT.
If you bring your calories up, you'll be able to exercise with higher intensity, to grow, and recover, faster. EAT.
You mention your stomach. Ever look at someone who is starving and what happens to their stomach (it's called distention)? There you go. EAT.
To answer your question directly: it's a silly question "How do you know when to "bulk up" vs get "lean muscle"?". If you like being small, you stay small. If you like being big, then, get big. There's not a time specified that you should get big, or get small.
I weigh 215 pounds at 5'5" and I'm certainly not fat. I haven't weighed 130 since I was in 7'th grade. I like being big, and strong, and lean. I like eating. It works for me. If you like being small, then, that's fine, too. You DO NOT NEED APPROVAL OF A GROUP TO MAKE THIS DECISION.
The answer is to do what you want.