Hello and hope you are doing well.
Your symptoms could be due to thrombophlebitis, which is inflammation of the vein. Check to see if the swelling is due to an abscess, in which case you may need antibiotics to tackle the infection. If there is fever, pain or redness this is more so. Please contact your concerned physician at the earliest. In the meantime, you can use OTC Acetaminophen for relief from pain. This is unlikely to cause gastritis.
Hope this helped and do keep us posted.