Am I entitled to a retroactive travel allowance?

If you are legally or contractually entitled to compensation from your employer, you are entitled to be paid retrospectively. How far back in time depends on what is regulated in the contract/law, but generally you have the right to be paid within two years after the event occurred.
Try to make a claim for compensation to the employer. If the employer refuses, get back to Naturvetarna's advice at radgivning@naturvetarna.se and we can take a closer look at the circumstances and see if we can help.