Why do i have a bit of an underbite sometimes and how can i fix it?

Complex question: That really can't be answered i. This forum. Your best bet is to see a dentist and discuss your issues.

Orthodontist: An orthodontist can correct an underbite, either with conventional braces or invisalign.

Orthodontia: This is not an uncommon malocclusion. If relatively minor as you note, it can be fixed through orthodontics typically without surgury.

Sometimes?: It sounds like it is not to severe. Consult an orthodontist or two and learn about your options.

Need evaluation: Having an underbite 'sometimes' may indicate that your lower jaw is not comfortable in its current arc of closure, which can result in a forward posturing to relief the discomfort. This 'posturing forward' could result in an underbite relationship. This is often the result of a confining relationship created by the front teeth(too straight up and down). Consult an orthodontist for an evaluation.

Underbite: Please see a fully qualified Orthodontic Specialist. Problem may be dental, skeletal, or both. Your description of your underbite worsening indicates deterioration of your occlusal relationships. Please see Orthodontist (initial exam usually free) for advice.