As an previous standardbred owner, I can assure you that for the first three month a mare will be 'full of beans' and often race better then ever. I don't know about light tack horses that are riden, but I would imagine they are the same. Your horse may settle down very soon and, as long as you show consideration for her condition, there should be no reason why you can't continue to do light work for the next 5-6months. As you probably know, every horse has a different temperament, so yours may not like being riden while in gestation, or she couldn't care less. Its up to you to notice her mood.