Last month i had a pea-sized bump on my neck. now it has been a bit smaller. it doesn't hurt but i hasn't gone away. is it serious?

Prob a lymph node: This prob is a lymph node. They become large when they filter out a virus or bacteria. When enlarged, it usually will take 6 weeks for the enlargement to go away. If the "bump" continues to be present or increases in size, then you need to be seen for eval by your health care provider.

Unlikely: Lymph nodes are common in the neck area, as long as they are very tiny, less than 1/2 cm. If they increase in size, they need to be looked at. Any swelling that increases in size over a 1- 2 week period needs assessment. In your case, you stated that it got smaller, hence my comment above. If in doubt, would still talk to your doctor and have it seen and felt. Better to be safe.