What were the investigations (blood work and imaging) done?
There is nothing to be scared, especially when just in October all your investigations were normal. The commonest reasons for this bleeding is hormonal aberrations / abnormalities. The other reasons can be endocervical polyp, ovarian cyst, uterine polyp, endometrial hyperplasia or uterine fibroid. With you ultrasound being normal; these have far less chance to be the cause. Still a repeat of ultrasound or blood work may be done by your primary care physician.
Please feel free for your follow up questions.