According to your symptoms, you seem to be experiencing tightening along with little spotting. You need immediate medical attention as the cause needs to be diagnosed. See your doctor and get assessed. Meanwhile avoid all activity, take bed rest
in left lateral position, avoid intercourse and try to remain in peace. You may need a sonogram to assess the placental position and the fetal well-being. Most often such cramps subside with treatment with no further harm. Good luck.