Laughing isn't acceptable, but sometimes people just can't help it, but omitting an apology to someone who was upset is inexcusable.
First of all, check your BMI calculation is correct. The normal range for adults is 18.5 - 25. Many people think it's 20 - 25, because other categrories go up in 5 unit blocks. Because height is squared, if it's wrong, particularly if too high, then it's easy to falsely calulate a BMI lower than reality. So, insist on having your height measured properly - no socks or shoes, measure attached to wall, someone else observing you from side, so that angle of head is correct - most people look upwards, whereas if they look straight ahead they actually gain approx 1cm in height.
However, that's not really the problem - you are unhappy with your low weight. It is also ignores the rate of weight loss, which any health professional should take seriously - I'm not quite clear how fast you are losing weight, and how this correlates with your changes in treatment / diet / exercise? Particularly as there is some confusion about what type of diabetes you have, I suggest you make an appointment to see your GP and explain your concern over weight loss.
Tina is right about ways to increase calorie intake with normal food - we did similar with my grandad when he had cancer of oesophagus and could only swallow pureed food. In the end, the hospice couldn't believe how well nourished he was without supplements - all down to nursing care / knowledge by his daughter in law and one of his grandchildren. Not that I'm suggesting you have cancer of oesophagus, of course. I suspect the underlying issue may be that you haven't yet been properly diagnosed and thus aren't yet on the right treatment.
Especially in times when regular healthcare services are closed, then you might find it helpful to phone Samaritans, because they're always on the end of the line - see
http://www.samaritans.org/ 08457 90 90 90 or email
jo@samaritans.org
Hope you can make some progress today towards getting more help and support - you deserve it