53 year old female on pregnazone for asmatic bronchitis had a huge blood loss from her period. scared. is this a side affect?

No: I assume you mean Prednisone . But the heavy bleeding is probably part of the hormonal changes due to her age, not the medication

Yes it can be: At 53 heavy periods occur without the use of Prednisone as well. Check 25-oh vit d, if low restore to normal. Take 80 mg of iron for 3 months with vit c to enhance absorption. Avoid dairy, wheat, sugar and alcohol and eat healthy unprocessed food with lots of vegetables, get 8 hours of sleep, learn relaxation breathing ( see kelly mcgonigal) and exercise regularly to improve asthma.

Vaginal bleeding : Given your age, see your gynecologist prior to blaming prednisone. An endometrial carcinoma cannot be neglected.