Let's start with an obvious:
1. Your female has gone into heat again. There is something called split heats, although usually not so close in timing ie : one month apart
2. Your male has smelled other females who are either close to heat or actually in heat. Most terriers have a good sense of smell and those smells can travel for miles.
3. Most females do NOT want to be humped by male dogs. The snarling, lip curling and growling, are their way of telling the males keep away. And the older the bitch gets the more likely and quickly she will respond this way. They are called bitches for a reason
Your female is trying to teach this one to NOT hump me Nope noway don;t like it. And she won't stop until his behaviour changes.
Nb: real life today story to elaborate: At training today, a full female who my full male has not seen for awhile was there. he adores this female, always has, looks like he always will. Our trainer said because Magic man was showing a lot of interest in her, oh she's going into heat Oct. After about 5-8 times of pulling my male away from her, I said well Ilona you might want to rethink when she is going into heat. Una the female, took exactly one or two sniffs at her butt, and then it was snap, snarl, and growl at Magic. Magic hadn't even started to hump her yet. But every chance he got, he did his old male thing of posturing. Gawd do they ever look fantastic when they do that. Chest out, head high, muscles bulging, testosterone roaring. By the end of the hour Magic just had to look at her, and her lip curled. This is a dog that could do serious damage to him, if she took it into her head to do. Did my boy care? Nope not one little old whit.
4. You need to train your dog not to hump your female. That means every time he goes to do so you interfere. Lure him away with a treat. Put him on a lead and tug him away. Discipline him when he does the humping business. Preferably interfere before it gets to a hump.
just look at humping as any undesirable behaviour you want to curb/control. Then do the appropriate discipline.
You've already got other advice about spaying. Some links that maybe will pan out for inexpensive spay/neuter clinics, so I truly hope you can avail yourself of some of the suggestions here.