Sitting in Half Lotus Position at the end of hong-sau technique, when I exhaled and held the breath as much as is comfortable, something strange happened. I did not feel the need to breathe again and the top of my body began to spin counter clockwise faster and faster. I was scared and went back to breathe forcefully. I meditate using the SRF techniques for more than two years and is the first time this happens. I ask, is it normal? and what should I do? Should I keep breathlessness and allow the spin?
An experience such as you had is a very good thing. It is not unique, but as your past experience bears out, neither is it ordinary. The spinning was likely the result of your Kundalini energy beginning to rise, which needs to happen in order for one to grow spiritually. Kundalini rising should be a pleasant, enjoyable experience, not a scary one. However, even a positive thing can be a bit scary if it is extremely unfamiliar.
Swami Kriyananda recommended that, in a case such as this, you simply try to keep the body still so the experience stays as inward as possible, rather than getting translated into so much outward movement that it takes you out of the experience.
One more thing: Do not force the breath to stay out after you exhale at the end of Hong-Sau practice. Let it stay out as long as it wants to, but not longer. True breathlessness will happen on its own, not by force.
Best wishes for many more deep meditations.