I'm having a gastric and constipation. it caused light-headedness and neck strain. sometimes, it was also followed by anxiety attack. is it normal?

Common is not Normal: This may be common but if normal means nothing is wrong, no - it's not normal. You might just need more water or a look at your diet/exercise lifestyle. It could be a specific flu or bad food. Anxiety is best managed by some CBT to learn skills and it can really help. This anxiety may have done a good job in motivating you - that's what it was intended to be - a push to action! Best wishes!