qualm

Short definition from Wiktionary

Noun

1. 1
   A feeling of apprehension, doubt, fear etc.
2. 2
   A sudden sickly feeling; queasiness.
3. 3
   A prick of the conscience; a moral scruple, a pang of guilt.
4. 4
   Mortality; plague; pestilence.
5. 5
   A calamity or disaster.

Verb

1. 1
   To have a sickly feeling.