Beta HCG 3.8

I've done 6 iuis and my beta hcg result has always been 0 (or <.100 to be precise) for 5 of them. But last month's IuI, my beta came back at 3.8 on 14 dpiui/dpo. I know a beta under 5 is negative (and dr has confirmed so), and my period came just 1 or 2 days later so I know I wasn't pregnant. But I'm wondering what could've caused the beta to be slightly higher even if still negative. Is it possible that it implanted and just didn't stick? Or is 3.8 normal even when not pregnant?