In 2017 cash and personal property contributions were limited to 50% of your AGI. The excess can be carried forward to 2018. It sounds very much like your prior year return(s) took too much charitable deductions.
and Worksheet 2 therein.
2018 is different in that cash contributions are limited to 60% of AGI.
https://www.irs.gov/pub/irs-pdf/p526.pdf
