pain after s*x

bakit po kaya may pain or burning sensation ako na-nafeel nung nag wiwi ako after namin mag do ni hubby? i'm currently 37wks, low risk pregnancy. everytime tlga after namin mag do, may burning sensation...aroused nmn ako and hndi dn hard s*x since we're being careful din dahil nga mjo malaki n tyan ko.. hndi naman gnito bfore..