Anapanasati Buddhist meditation

So I thought I really need to ask this question about Buddhist meditation specifically, Anapanasati. Basically I sit cross legged and count breaths for 15 minutes as advised on the Internet for beginners. Correct me if I am wrong but I think I might be overdoing my breathing towards probably the last 5 minutes. The breathing becomes heavy and I work harder to take deeper breaths . And I feel some sort of stiffness in my mouth and neck. And at the end of 15 minutes, I wake? Or get up all giddy (probably due to the heavy breaths) and feeling something out of this world for a while like say a minute. Is this the way you're supposed to feel after doing a Anapanasati? Because it feels so odd probably due to the processes the brain undergoes during heavy breathing or rather forced heavy breathing. Would like some insights from meditation practitioners.