I'd love to help you out, but I'm a bit short on cash this month as well.
'be short on cash'就是手头紧的意思.在这里,short有"不足,短缺"之意.
Short: not having enough of something; lacking something 不足;短缺 not before noun ~ (of sth)
I'm afraid I'm a little short(= of money)this month. 这个月我恐怕手头有点紧.
She is not short of excuses when things go wrong. 事情出了差错,她老有借口.
还有一种说法是"be strapped for cash",也很常见.
I'm strapped for cash at the moment─can I pay you later? 我眼下现金紧张--以后付你可以吗?