A weight problem, and not the weight-loss kind...

I don't know where to put this, so please let me know the subreddit if I'm misdirected (I looked around and couldn't find a good reddit to put this on as it is...).

A little more than two years ago, I was severely depressed and lost a lot of weight, unhealthily. I went from weighing something pretty healthy (130 pounds and 5'6") to something pretty unhealthy (110-115 pounds). I've recently intended to gain a little back. I made about 120 my goal, and I've been trying to gain it back healthily by making sure I eat enough and exercise moderately so I'm building muscle, not fat.

I came home from school and went about my usual routine, weighing myself to make sure that I hadn't lost any weight throughout the semester and hoping that I had gained some. I was really proud. My bathroom scale said I gained almost 3 pounds.

However, I recently found out that scale is wrong, and I weigh about 110. That's almost five pounds underweight. It's been a huge emotional setback. I'm wondering if I have some sort of delusion that's causing me to think I'm eating more than I actually am--or some sort of medical issue that is causing me to have trouble gaining back weight I lost.

For those who are interested, the weight loss was caused by inadequate diet (depression kills the appetite for me). I figured that gradually increasing the amount of food I ate while keeping exercise consistent would be a recipe for healthy weight gain. Other than that, there hasn't been anything medically significant that would cause me to have problems gaining weight.

I'm getting consistently more sad. There's a lot of people harping on me about how skinny I am, and a lot of them explicitly calling me anorexic. I'm genuinely concerned with the health problems I could have being underweight, and I'm willing to eat as much as I need to be at a healthy weight. To try so hard and then have people turn around and dismiss my effort is very disappointing.

Has anyone been in a similar situation? What should I do? When should I mention this to a doctor?

tldr; Lost a lot of weight, and now I can't gain it back despite my concerted efforts. What should I do?